git/pack-bitmap.h

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pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
#ifndef PACK_BITMAP_H
#define PACK_BITMAP_H
#include "ewah/ewok.h"
#include "khash.h"
#include "pack.h"
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
#include "pack-objects.h"
builtin/pack-objects.c: respect 'pack.preferBitmapTips' When writing a new pack with a bitmap, it is sometimes convenient to indicate some reference prefixes which should receive priority when selecting which commits to receive bitmaps. A truly motivated caller could accomplish this by setting 'pack.islandCore', (since all commits in the core island are similarly marked as preferred) but this requires callers to opt into using delta islands, which they may or may not want to do. Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to allow callers to specify a list of reference prefixes. All references which have a prefix contained in 'pack.preferBitmapTips' will mark their tips as "preferred" in the same way as commits are marked as preferred for selection by 'pack.islandCore'. The choice of the verb "prefer" is intentional: marking the NEEDS_BITMAP flag on an object does *not* guarantee that that object will receive a bitmap. It merely guarantees that that commit will receive a bitmap over any *other* commit in the same window by bitmap_writer_select_commits(). The test this patch adds reflects this quirk, too. It only tests that a commit (which didn't receive bitmaps by default) is selected for bitmaps after changing the value of 'pack.preferBitmapTips' to include it. Other commits may lose their bitmaps as a byproduct of how the selection process works (bitmap_writer_select_commits() ignores the remainder of a window after seeing a commit with the NEEDS_BITMAP flag). This configuration will aide in selecting important references for multi-pack bitmaps, since they do not respect the same pack.islandCore configuration. (They could, but doing so may be confusing, since it is packs--not bitmaps--which are influenced by the delta-islands configuration). In a fork network repository (one which lists all forks of a given repository as remotes), for example, it is useful to set pack.preferBitmapTips to 'refs/remotes/<root>/heads' and 'refs/remotes/<root>/tags', where '<root>' is an opaque identifier referring to the repository which is at the base of the fork chain. Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-04-01 01:32:14 +00:00
#include "string-list.h"
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
struct commit;
struct repository;
struct rev_info;
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static const char BITMAP_IDX_SIGNATURE[] = {'B', 'I', 'T', 'M'};
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
struct bitmap_disk_header {
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char magic[ARRAY_SIZE(BITMAP_IDX_SIGNATURE)];
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
uint16_t version;
uint16_t options;
uint32_t entry_count;
unsigned char checksum[GIT_MAX_RAWSZ];
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
};
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
#define NEEDS_BITMAP (1u<<22)
/*
* The width in bytes of a single triplet in the lookup table
* extension:
* (commit_pos, offset, xor_row)
*
* whose fields ar 32-, 64-, 32- bits wide, respectively.
*/
#define BITMAP_LOOKUP_TABLE_TRIPLET_WIDTH (16)
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
enum pack_bitmap_opts {
BITMAP_OPT_FULL_DAG = 0x1,
BITMAP_OPT_HASH_CACHE = 0x4,
BITMAP_OPT_LOOKUP_TABLE = 0x10,
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
};
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
enum pack_bitmap_flags {
BITMAP_FLAG_REUSE = 0x1
};
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
typedef int (*show_reachable_fn)(
const struct object_id *oid,
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
enum object_type type,
int flags,
uint32_t hash,
struct packed_git *found_pack,
off_t found_offset);
struct bitmap_index;
midx: implement `BTMP` chunk When a multi-pack bitmap is used to implement verbatim pack reuse (that is, when verbatim chunks from an on-disk packfile are copied directly[^1]), it does so by using its "preferred pack" as the source for pack-reuse. This allows repositories to pack the majority of their objects into a single (often large) pack, and then use it as the single source for verbatim pack reuse. This increases the amount of objects that are reused verbatim (and consequently, decrease the amount of time it takes to generate many packs). But this performance comes at a cost, which is that the preferred packfile must pace its growth with that of the entire repository in order to maintain the utility of verbatim pack reuse. As repositories grow beyond what we can reasonably store in a single packfile, the utility of verbatim pack reuse diminishes. Or, at the very least, it becomes increasingly more expensive to maintain as the pack grows larger and larger. It would be beneficial to be able to perform this same optimization over multiple packs, provided some modest constraints (most importantly, that the set of packs eligible for verbatim reuse are disjoint with respect to the subset of their objects being sent). If we assume that the packs which we treat as candidates for verbatim reuse are disjoint with respect to any of their objects we may output, we need to make only modest modifications to the verbatim pack-reuse code itself. Most notably, we need to remove the assumption that the bits in the reachability bitmap corresponding to objects from the single reuse pack begin at the first bit position. Future patches will unwind these assumptions and reimplement their existing functionality as special cases of the more general assumptions (e.g. that reuse bits can start anywhere within the bitset, but happen to start at 0 for all existing cases). This patch does not yet relax any of those assumptions. Instead, it implements a foundational data-structure, the "Bitampped Packs" (`BTMP`) chunk of the multi-pack index. The `BTMP` chunk's contents are described in detail here. Importantly, the `BTMP` chunk contains information to map regions of a multi-pack index's reachability bitmap to the packs whose objects they represent. For now, this chunk is only written, not read (outside of the test-tool used in this patch to test the new chunk's behavior). Future patches will begin to make use of this new chunk. [^1]: Modulo patching any `OFS_DELTA`'s that cross over a region of the pack that wasn't used verbatim. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-12-14 22:23:51 +00:00
struct bitmapped_pack {
struct packed_git *p;
uint32_t bitmap_pos;
uint32_t bitmap_nr;
uint32_t pack_int_id; /* MIDX only */
};
struct bitmap_index *prepare_bitmap_git(struct repository *r);
struct bitmap_index *prepare_midx_bitmap_git(struct multi_pack_index *midx);
void count_bitmap_commit_list(struct bitmap_index *, uint32_t *commits,
uint32_t *trees, uint32_t *blobs, uint32_t *tags);
void traverse_bitmap_commit_list(struct bitmap_index *,
rev-list: allow commit-only bitmap traversals Ever since we added reachability bitmap support, we've been able to use it with rev-list to get the full list of objects, like: git rev-list --objects --use-bitmap-index --all But you can't do so without --objects, since we weren't ready to just show the commits. However, the internals of the bitmap code are mostly ready for this: they avoid opening up trees when walking to fill in the bitmaps. We just need to actually pass in the rev_info to traverse_bitmap_commit_list() so it knows which types to bother triggering our callback for. For completeness, the perf test now covers both the existing --objects case, as well as the new commits-only behavior (the objects one got way faster when we introduced bitmaps, but obviously isn't improved now). Here are numbers for linux.git: Test HEAD^ HEAD ------------------------------------------------------------------------ 5310.7: rev-list (commits) 8.29(8.10+0.19) 1.76(1.72+0.04) -78.8% 5310.8: rev-list (objects) 8.06(7.94+0.12) 8.14(7.94+0.13) +1.0% That run was cheating a little, as I didn't have any commit-graph in the repository, and we'd built it by default these days when running git-gc. Here are numbers with a commit-graph: Test HEAD^ HEAD ------------------------------------------------------------------------ 5310.7: rev-list (commits) 0.70(0.58+0.12) 0.51(0.46+0.04) -27.1% 5310.8: rev-list (objects) 6.20(6.09+0.10) 6.27(6.16+0.11) +1.1% Still an improvement, but a lot less impressive. We could have the perf script remove any commit-graph to show the out-sized effect, but it probably makes sense to leave it in what would be a more typical setup. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-02-14 18:22:27 +00:00
struct rev_info *revs,
show_reachable_fn show_reachable);
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
void test_bitmap_walk(struct rev_info *revs);
int test_bitmap_commits(struct repository *r);
int test_bitmap_hashes(struct repository *r);
pack-bitmap.c: use commit boundary during bitmap traversal When reachability bitmap coverage exists in a repository, Git will use a different (and hopefully faster) traversal to compute revision walks. Consider a set of positive and negative tips (which we'll refer to with their standard bitmap parlance by "wants", and "haves"). In order to figure out what objects exist between the tips, the existing traversal in `prepare_bitmap_walk()` does something like: 1. Consider if we can even compute the set of objects with bitmaps, and fall back to the usual traversal if we cannot. For example, pathspec limiting traversals can't be computed using bitmaps (since they don't know which objects are at which paths). The same is true of certain kinds of non-trivial object filters. 2. If we can compute the traversal with bitmaps, partition the (dereferenced) tips into two object lists, "haves", and "wants", based on whether or not the objects have the UNINTERESTING flag, respectively. 3. Fall back to the ordinary object traversal if either (a) there are more than zero haves, none of which are in the bitmapped pack or MIDX, or (b) there are no wants. 4. Construct a reachability bitmap for the "haves" side by walking from the revision tips down to any existing bitmaps, OR-ing in any bitmaps as they are found. 5. Then do the same for the "wants" side, stopping at any objects that appear in the "haves" bitmap. 6. Filter the results if any object filter (that can be easily computed with bitmaps alone) was given, and then return back to the caller. When there is good bitmap coverage relative to the traversal tips, this walk is often significantly faster than an ordinary object traversal because it can visit far fewer objects. But in certain cases, it can be significantly *slower* than the usual object traversal. Why? Because we need to compute complete bitmaps on either side of the walk. If either one (or both) of the sides require walking many (or all!) objects before they get to an existing bitmap, the extra bitmap machinery is mostly or all overhead. One of the benefits, however, is that even if the walk is slower, bitmap traversals are guaranteed to provide an *exact* answer. Unlike the traditional object traversal algorithm, which can over-count the results by not opening trees for older commits, the bitmap walk builds an exact reachability bitmap for either side, meaning the results are never over-counted. But producing non-exact results is OK for our traversal here (both in the bitmap case and not), as long as the results are over-counted, not under. Relaxing the bitmap traversal to allow it to produce over-counted results gives us the opportunity to make some significant improvements. Instead of the above, the new algorithm only has to walk from the *boundary* down to the nearest bitmap, instead of from each of the UNINTERESTING tips. The boundary-based approach still has degenerate cases, but we'll show in a moment that it is often a significant improvement. The new algorithm works as follows: 1. Build a (partial) bitmap of the haves side by first OR-ing any bitmap(s) that already exist for UNINTERESTING commits between the haves and the boundary. 2. For each commit along the boundary, add it as a fill-in traversal tip (where the traversal terminates once an existing bitmap is found), and perform fill-in traversal. 3. Build up a complete bitmap of the wants side as usual, stopping any time we intersect the (partial) haves side. 4. Return the results. And is more-or-less equivalent to using the *old* algorithm with this invocation: $ git rev-list --objects --use-bitmap-index $WANTS --not \ $(git rev-list --objects --boundary $WANTS --not $HAVES | perl -lne 'print $1 if /^-(.*)/') The new result performs significantly better in many cases, particularly when the distance from the boundary commit(s) to an existing bitmap is shorter than the distance from (all of) the have tips to the nearest bitmapped commit. Note that when using the old bitmap traversal algorithm, the results can be *slower* than without bitmaps! Under the new algorithm, the result is computed faster with bitmaps than without (at the cost of over-counting the true number of objects in a similar fashion as the non-bitmap traversal): # (Computing the number of tagged objects not on any branches # without bitmaps). $ time git rev-list --count --objects --tags --not --branches 20 real 0m1.388s user 0m1.092s sys 0m0.296s # (Computing the same query using the old bitmap traversal). $ time git rev-list --count --objects --tags --not --branches --use-bitmap-index 19 real 0m22.709s user 0m21.628s sys 0m1.076s # (this commit) $ time git.compile rev-list --count --objects --tags --not --branches --use-bitmap-index 19 real 0m1.518s user 0m1.234s sys 0m0.284s The new algorithm is still slower than not using bitmaps at all, but it is nearly a 15-fold improvement over the existing traversal. In a more realistic setting (using my local copy of git.git), I can observe a similar (if more modest) speed-up: $ argv="--count --objects --branches --not --tags" hyperfine \ -n 'no bitmaps' "git.compile rev-list $argv" \ -n 'existing traversal' "git.compile rev-list --use-bitmap-index $argv" \ -n 'boundary traversal' "git.compile -c pack.useBitmapBoundaryTraversal=true rev-list --use-bitmap-index $argv" Benchmark 1: no bitmaps Time (mean ± σ): 124.6 ms ± 2.1 ms [User: 103.7 ms, System: 20.8 ms] Range (min … max): 122.6 ms … 133.1 ms 22 runs Benchmark 2: existing traversal Time (mean ± σ): 368.6 ms ± 3.0 ms [User: 325.3 ms, System: 43.1 ms] Range (min … max): 365.1 ms … 374.8 ms 10 runs Benchmark 3: boundary traversal Time (mean ± σ): 167.6 ms ± 0.9 ms [User: 139.5 ms, System: 27.9 ms] Range (min … max): 166.1 ms … 169.2 ms 17 runs Summary 'no bitmaps' ran 1.34 ± 0.02 times faster than 'boundary traversal' 2.96 ± 0.05 times faster than 'existing traversal' Here, the new algorithm is also still slower than not using bitmaps, but represents a more than 2-fold improvement over the existing traversal in a more modest example. Since this algorithm was originally written (nearly a year and a half ago, at the time of writing), the bitmap lookup table shipped, making the new algorithm's result more competitive. A few other future directions for improving bitmap traversal times beyond not using bitmaps at all: - Decrease the cost to decompress and OR together many bitmaps together (particularly when enumerating the uninteresting side of the walk). Here we could explore more efficient bitmap storage techniques, like Roaring+Run and/or use SIMD instructions to speed up ORing them together. - Store pseudo-merge bitmaps, which could allow us to OR together fewer "summary" bitmaps (which would also help with the above). Helped-by: Jeff King <peff@peff.net> Helped-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-05-08 17:38:12 +00:00
#define GIT_TEST_PACK_USE_BITMAP_BOUNDARY_TRAVERSAL \
"GIT_TEST_PACK_USE_BITMAP_BOUNDARY_TRAVERSAL"
struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs,
int filter_provided_objects);
void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
struct bitmapped_pack **packs_out,
size_t *packs_nr_out,
struct bitmap **reuse_out);
int rebuild_existing_bitmaps(struct bitmap_index *, struct packing_data *mapping,
kh_oid_map_t *reused_bitmaps, int show_progress);
void free_bitmap_index(struct bitmap_index *);
int bitmap_walk_contains(struct bitmap_index *,
struct bitmap *bitmap, const struct object_id *oid);
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
/*
bitmap_has_sha1_in_uninteresting(): drop BUG check Commit 30cdc33fba (pack-bitmap: save "have" bitmap from walk, 2018-08-21) introduced a new function for looking at the "have" side of a bitmap walk. Because it only makes sense to do so after we've finished the walk, we added an extra safety assertion, making sure that bitmap_git->result is non-NULL. However, this safety is misguided. It was trying to catch the case where we had called prepare_bitmap_walk() to give us a "struct bitmap_index", but had not yet called traverse_bitmap_commit_list() to walk it. But all of the interesting computation (including setting up the result and "have" bitmaps) happens in the first function! The latter function only delivers the result to a callback function. So the case we were worried about is impossible; if you get a non-NULL result from prepare_bitmap_walk(), then its "have" field will be fully formed. But much worse, traverse_bitmap_commit_list() actually frees the result field as it finishes. Which means that this assertion is worse than useless: it's almost guaranteed to trigger! Our test suite didn't catch this because the function isn't actually exercised at all. The only caller comes from 6a1e32d532 (pack-objects: reuse on-disk deltas for thin "have" objects, 2018-08-21), and that's triggered only when you fetch or push history that contains an object with a base that is found deep in history. Our test suite fetches and pushes either don't use bitmaps, or use too-small example repositories. But any reasonably-sized real-world push or fetch (with bitmaps) would trigger this. This patch drops the harmful assertion and tweaks the docstring for the function to make the precondition clear. The tests need to be improved to exercise this new pack-objects feature, but we'll do that in a separate commit. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-09-01 07:44:48 +00:00
* After a traversal has been performed by prepare_bitmap_walk(), this can be
* queried to see if a particular object was reachable from any of the
* objects flagged as UNINTERESTING.
*/
int bitmap_has_oid_in_uninteresting(struct bitmap_index *, const struct object_id *oid);
rev-list: add --disk-usage option for calculating disk usage It can sometimes be useful to see which refs are contributing to the overall repository size (e.g., does some branch have a bunch of objects not found elsewhere in history, which indicates that deleting it would shrink the size of a clone). You can find that out by generating a list of objects, getting their sizes from cat-file, and then summing them, like: git rev-list --objects --no-object-names main..branch git cat-file --batch-check='%(objectsize:disk)' | perl -lne '$total += $_; END { print $total }' Though note that the caveats from git-cat-file(1) apply here. We "blame" base objects more than their deltas, even though the relationship could easily be flipped. Still, it can be a useful rough measure. But one problem is that it's slow to run. Teaching rev-list to sum up the sizes can be much faster for two reasons: 1. It skips all of the piping of object names and sizes. 2. If bitmaps are in use, for objects that are in the bitmapped packfile we can skip the oid_object_info() lookup entirely, and just ask the revindex for the on-disk size. This patch implements a --disk-usage option which produces the same answer in a fraction of the time. Here are some timings using a clone of torvalds/linux: [rev-list piped to cat-file, no bitmaps] $ time git rev-list --objects --no-object-names --all | git cat-file --buffer --batch-check='%(objectsize:disk)' | perl -lne '$total += $_; END { print $total }' 1459938510 real 0m29.635s user 0m38.003s sys 0m1.093s [internal, no bitmaps] $ time git rev-list --disk-usage --objects --all 1459938510 real 0m31.262s user 0m30.885s sys 0m0.376s Even though the wall-clock time is slightly worse due to parallelism, notice the CPU savings between the two. We saved 21% of the CPU just by avoiding the pipes. But the real win is with bitmaps. If we use them without the new option: [rev-list piped to cat-file, bitmaps] $ time git rev-list --objects --no-object-names --all --use-bitmap-index | git cat-file --batch-check='%(objectsize:disk)' | perl -lne '$total += $_; END { print $total }' 1459938510 real 0m6.244s user 0m8.452s sys 0m0.311s then we're faster to generate the list of objects, but we still spend a lot of time piping and looking things up. But if we do both together: [internal, bitmaps] $ time git rev-list --disk-usage --objects --all --use-bitmap-index 1459938510 real 0m0.219s user 0m0.169s sys 0m0.049s then we get the same answer much faster. For "--all", that answer will correspond closely to "du objects/pack", of course. But we're actually checking reachability here, so we're still fast when we ask for more interesting things: $ time git rev-list --disk-usage --use-bitmap-index v5.0..v5.10 374798628 real 0m0.429s user 0m0.356s sys 0m0.072s Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-02-09 10:53:50 +00:00
off_t get_disk_usage_from_bitmap(struct bitmap_index *, struct rev_info *);
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
void bitmap_writer_show_progress(int show);
void bitmap_writer_set_checksum(const unsigned char *sha1);
void bitmap_writer_build_type_index(struct packing_data *to_pack,
struct pack_idx_entry **index,
uint32_t index_nr);
pack-bitmap-write: ignore BITMAP_FLAG_REUSE The on-disk bitmap format has a flag to mark a bitmap to be "reused". This is a rather curious feature, and works like this: - a run of pack-objects would decide to mark the last 80% of the bitmaps it generates with the reuse flag - the next time we generate bitmaps, we'd see those reuse flags from the last run, and mark those commits as special: - we'd be more likely to select those commits to get bitmaps in the new output - when generating the bitmap for a selected commit, we'd reuse the old bitmap as-is (rearranging the bits to match the new pack, of course) However, neither of these behaviors particularly makes sense. Just because a commit happened to be bitmapped last time does not make it a good candidate for having a bitmap this time. In particular, we may choose bitmaps based on how recent they are in history, or whether a ref tip points to them, and those things will change. We're better off re-considering fresh which commits are good candidates. Reusing the existing bitmap _is_ a reasonable thing to do to save computation. But only reusing exact bitmaps is a weak form of this. If we have an old bitmap for A and now want a new bitmap for its child, we should be able to compute that only by looking at trees and that are new to the child. But this code would consider only exact reuse (which is perhaps why it was eager to select those commits in the first place). Furthermore, the recent switch to the reverse-edge algorithm for generating bitmaps dropped this optimization entirely (and yet still performs better). So let's do a few cleanups: - drop the whole "reusing bitmaps" phase of generating bitmaps. It's not helping anything, and is mostly unused code (or worse, code that is using CPU but not doing anything useful) - drop the use of the on-disk reuse flag to select commits to bitmap - stop setting the on-disk reuse flag in bitmaps we generate (since nothing respects it anymore) We will keep a few innards of the reuse code, which will help us implement a more capable version of the "reuse" optimization: - simplify rebuild_existing_bitmaps() into a function that only builds the mapping of bits between the old and new orders, but doesn't actually convert any bitmaps - make rebuild_bitmap() public; we'll call it lazily to convert bitmaps as we traverse (using the mapping created above) Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-12-08 22:04:34 +00:00
uint32_t *create_bitmap_mapping(struct bitmap_index *bitmap_git,
struct packing_data *mapping);
int rebuild_bitmap(const uint32_t *reposition,
struct ewah_bitmap *source,
struct bitmap *dest);
struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
struct commit *commit);
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
void bitmap_writer_select_commits(struct commit **indexed_commits,
unsigned int indexed_commits_nr, int max_bitmaps);
int bitmap_writer_build(struct packing_data *to_pack);
pack-objects: implement bitmap writing This commit extends more the functionality of `pack-objects` by allowing it to write out a `.bitmap` index next to any written packs, together with the `.idx` index that currently gets written. If bitmap writing is enabled for a given repository (either by calling `pack-objects` with the `--write-bitmap-index` flag or by having `pack.writebitmaps` set to `true` in the config) and pack-objects is writing a packfile that would normally be indexed (i.e. not piping to stdout), we will attempt to write the corresponding bitmap index for the packfile. Bitmap index writing happens after the packfile and its index has been successfully written to disk (`finish_tmp_packfile`). The process is performed in several steps: 1. `bitmap_writer_set_checksum`: this call stores the partial checksum for the packfile being written; the checksum will be written in the resulting bitmap index to verify its integrity 2. `bitmap_writer_build_type_index`: this call uses the array of `struct object_entry` that has just been sorted when writing out the actual packfile index to disk to generate 4 type-index bitmaps (one for each object type). These bitmaps have their nth bit set if the given object is of the bitmap's type. E.g. the nth bit of the Commits bitmap will be 1 if the nth object in the packfile index is a commit. This is a very cheap operation because the bitmap writing code has access to the metadata stored in the `struct object_entry` array, and hence the real type for each object in the packfile. 3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap index for one of the packfiles we're trying to repack, this call will efficiently rebuild the existing bitmaps so they can be reused on the new index. All the existing bitmaps will be stored in a `reuse` hash table, and the commit selection phase will prioritize these when selecting, as they can be written directly to the new index without having to perform a revision walk to fill the bitmap. This can greatly speed up the repack of a repository that already has bitmaps. 4. `bitmap_writer_select_commits`: if bitmap writing is enabled for a given `pack-objects` run, the sequence of commits generated during the Counting Objects phase will be stored in an array. We then use that array to build up the list of selected commits. Writing a bitmap in the index for each object in the repository would be cost-prohibitive, so we use a simple heuristic to pick the commits that will be indexed with bitmaps. The current heuristics are a simplified version of JGit's original implementation. We select a higher density of commits depending on their age: the 100 most recent commits are always selected, after that we pick 1 commit of each 100, and the gap increases as the commits grow older. On top of that, we make sure that every single branch that has not been merged (all the tips that would be required from a clone) gets their own bitmap, and when selecting commits between a gap, we tend to prioritize the commit with the most parents. Do note that there is no right/wrong way to perform commit selection; different selection algorithms will result in different commits being selected, but there's no such thing as "missing a commit". The bitmap walker algorithm implemented in `prepare_bitmap_walk` is able to adapt to missing bitmaps by performing manual walks that complete the bitmap: the ideal selection algorithm, however, would select the commits that are more likely to be used as roots for a walk in the future (e.g. the tips of each branch, and so on) to ensure a bitmap for them is always available. 5. `bitmap_writer_build`: this is the computationally expensive part of bitmap generation. Based on the list of commits that were selected in the previous step, we perform several incremental walks to generate the bitmap for each commit. The walks begin from the oldest commit, and are built up incrementally for each branch. E.g. consider this dag where A, B, C, D, E, F are the selected commits, and a, b, c, e are a chunk of simplified history that will not receive bitmaps. A---a---B--b--C--c--D \ E--e--F We start by building the bitmap for A, using A as the root for a revision walk and marking all the objects that are reachable until the walk is over. Once this bitmap is stored, we reuse the bitmap walker to perform the walk for B, assuming that once we reach A again, the walk will be terminated because A has already been SEEN on the previous walk. This process is repeated for C, and D, but when we try to generate the bitmaps for E, we can reuse neither the current walk nor the bitmap we have generated so far. What we do now is resetting both the walk and clearing the bitmap, and performing the walk from scratch using E as the origin. This new walk, however, does not need to be completed. Once we hit B, we can lookup the bitmap we have already stored for that commit and OR it with the existing bitmap we've composed so far, allowing us to limit the walk early. After all the bitmaps have been generated, another iteration through the list of commits is performed to find the best XOR offsets for compression before writing them to disk. Because of the incremental nature of these bitmaps, XORing one of them with its predecesor results in a minimal "bitmap delta" most of the time. We can write this delta to the on-disk bitmap index, and then re-compose the original bitmaps by XORing them again when loaded. This is a phase very similar to pack-object's `find_delta` (using bitmaps instead of objects, of course), except the heuristics have been greatly simplified: we only check the 10 bitmaps before any given one to find best compressing one. This gives good results in practice, because there is locality in the ordering of the objects (and therefore bitmaps) in the packfile. 6. `bitmap_writer_finish`: the last step in the process is serializing to disk all the bitmap data that has been generated in the two previous steps. The bitmap is written to a tmp file and then moved atomically to its final destination, using the same process as `pack-write.c:write_idx_file`. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:16 +00:00
void bitmap_writer_finish(struct pack_idx_entry **index,
uint32_t index_nr,
pack-bitmap: implement optional name_hash cache When we use pack bitmaps rather than walking the object graph, we end up with the list of objects to include in the packfile, but we do not know the path at which any tree or blob objects would be found. In a recently packed repository, this is fine. A fetch would use the paths only as a heuristic in the delta compression phase, and a fully packed repository should not need to do much delta compression. As time passes, though, we may acquire more objects on top of our large bitmapped pack. If clients fetch frequently, then they never even look at the bitmapped history, and all works as usual. However, a client who has not fetched since the last bitmap repack will have "have" tips in the bitmapped history, but "want" newer objects. The bitmaps themselves degrade gracefully in this circumstance. We manually walk the more recent bits of history, and then use bitmaps when we hit them. But we would also like to perform delta compression between the newer objects and the bitmapped objects (both to delta against what we know the user already has, but also between "new" and "old" objects that the user is fetching). The lack of pathnames makes our delta heuristics much less effective. This patch adds an optional cache of the 32-bit name_hash values to the end of the bitmap file. If present, a reader can use it to match bitmapped and non-bitmapped names during delta compression. Here are perf results for p5310: Test origin/master HEAD^ HEAD ------------------------------------------------------------------------------------------------- 5310.2: repack to disk 36.81(37.82+1.43) 47.70(48.74+1.41) +29.6% 47.75(48.70+1.51) +29.7% 5310.3: simulated clone 30.78(29.70+2.14) 1.08(0.97+0.10) -96.5% 1.07(0.94+0.12) -96.5% 5310.4: simulated fetch 3.16(6.10+0.08) 3.54(10.65+0.06) +12.0% 1.70(3.07+0.06) -46.2% 5310.6: partial bitmap 36.76(43.19+1.81) 6.71(11.25+0.76) -81.7% 4.08(6.26+0.46) -88.9% You can see that the time spent on an incremental fetch goes down, as our delta heuristics are able to do their work. And we save time on the partial bitmap clone for the same reason. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:45 +00:00
const char *filename,
uint16_t options);
char *midx_bitmap_filename(struct multi_pack_index *midx);
char *pack_bitmap_filename(struct packed_git *p);
int bitmap_is_midx(struct bitmap_index *bitmap_git);
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
builtin/pack-objects.c: respect 'pack.preferBitmapTips' When writing a new pack with a bitmap, it is sometimes convenient to indicate some reference prefixes which should receive priority when selecting which commits to receive bitmaps. A truly motivated caller could accomplish this by setting 'pack.islandCore', (since all commits in the core island are similarly marked as preferred) but this requires callers to opt into using delta islands, which they may or may not want to do. Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to allow callers to specify a list of reference prefixes. All references which have a prefix contained in 'pack.preferBitmapTips' will mark their tips as "preferred" in the same way as commits are marked as preferred for selection by 'pack.islandCore'. The choice of the verb "prefer" is intentional: marking the NEEDS_BITMAP flag on an object does *not* guarantee that that object will receive a bitmap. It merely guarantees that that commit will receive a bitmap over any *other* commit in the same window by bitmap_writer_select_commits(). The test this patch adds reflects this quirk, too. It only tests that a commit (which didn't receive bitmaps by default) is selected for bitmaps after changing the value of 'pack.preferBitmapTips' to include it. Other commits may lose their bitmaps as a byproduct of how the selection process works (bitmap_writer_select_commits() ignores the remainder of a window after seeing a commit with the NEEDS_BITMAP flag). This configuration will aide in selecting important references for multi-pack bitmaps, since they do not respect the same pack.islandCore configuration. (They could, but doing so may be confusing, since it is packs--not bitmaps--which are influenced by the delta-islands configuration). In a fork network repository (one which lists all forks of a given repository as remotes), for example, it is useful to set pack.preferBitmapTips to 'refs/remotes/<root>/heads' and 'refs/remotes/<root>/tags', where '<root>' is an opaque identifier referring to the repository which is at the base of the fork chain. Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-04-01 01:32:14 +00:00
const struct string_list *bitmap_preferred_tips(struct repository *r);
int bitmap_is_preferred_refname(struct repository *r, const char *refname);
builtin/pack-objects.c: respect 'pack.preferBitmapTips' When writing a new pack with a bitmap, it is sometimes convenient to indicate some reference prefixes which should receive priority when selecting which commits to receive bitmaps. A truly motivated caller could accomplish this by setting 'pack.islandCore', (since all commits in the core island are similarly marked as preferred) but this requires callers to opt into using delta islands, which they may or may not want to do. Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to allow callers to specify a list of reference prefixes. All references which have a prefix contained in 'pack.preferBitmapTips' will mark their tips as "preferred" in the same way as commits are marked as preferred for selection by 'pack.islandCore'. The choice of the verb "prefer" is intentional: marking the NEEDS_BITMAP flag on an object does *not* guarantee that that object will receive a bitmap. It merely guarantees that that commit will receive a bitmap over any *other* commit in the same window by bitmap_writer_select_commits(). The test this patch adds reflects this quirk, too. It only tests that a commit (which didn't receive bitmaps by default) is selected for bitmaps after changing the value of 'pack.preferBitmapTips' to include it. Other commits may lose their bitmaps as a byproduct of how the selection process works (bitmap_writer_select_commits() ignores the remainder of a window after seeing a commit with the NEEDS_BITMAP flag). This configuration will aide in selecting important references for multi-pack bitmaps, since they do not respect the same pack.islandCore configuration. (They could, but doing so may be confusing, since it is packs--not bitmaps--which are influenced by the delta-islands configuration). In a fork network repository (one which lists all forks of a given repository as remotes), for example, it is useful to set pack.preferBitmapTips to 'refs/remotes/<root>/heads' and 'refs/remotes/<root>/tags', where '<root>' is an opaque identifier referring to the repository which is at the base of the fork chain. Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-04-01 01:32:14 +00:00
fsck: verify checksums of all .bitmap files If a filesystem-level corruption occurs in a .bitmap file, Git can react poorly. This could take the form of a run-time error due to failing to parse an EWAH bitmap or be more subtle such as returning the wrong set of objects to a fetch or clone. A natural first response to either of these kinds of errors is to run 'git fsck' to see if any files are corrupt. This currently ignores all .bitmap files. Add checks to 'git fsck' for all .bitmap files that are currently associated with a multi-pack-index or pack file. Verify their checksums using the hashfile API. We iterate through all multi-pack-indexes and pack-files to be sure to check all .bitmap files, not just the one that would be read by the process. For example, a multi-pack-index bitmap overrules a pack-bitmap. However, if the multi-pack-index is removed, the pack-bitmap may be selected instead. Be thorough to include every file that could become active in such a way. This includes checking files in alternates. There is potential that we could extend this effort to check the structure of the reachability bitmaps themselves, but it is very expensive to do so. At minimum, it's as expensive as generating the bitmaps in the first place, and that's assuming that we don't use the trivial algorithm of verifying each bitmap individually. The trivial algorithm will result in quadratic behavior (number of objects times number of bitmapped commits) while the bitmap building operation constructs a lattice of commits to build bitmaps incrementally and then generate the final bitmaps from a subset of those commits. If we were to extend 'git fsck' to check .bitmap file contents more closely like this, then we would likely want to hide it behind an option that signals the user is more willing to do expensive operations such as this. For testing, set up a repository with a pack-bitmap _and_ a multi-pack-index bitmap. This requires some file movement to avoid deleting the pack-bitmap during the repack that creates the multi-pack-index bitmap. We can then verify that 'git fsck' is checking all files, not just the "active" bitmap. Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-05-02 13:27:21 +00:00
int verify_bitmap_files(struct repository *r);
pack-bitmap: add support for bitmap indexes A bitmap index is a `.bitmap` file that can be found inside `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and contains precalculated reachability information for selected commits. The full specification of the format for these bitmap indexes can be found in `Documentation/technical/bitmap-format.txt`. For a given commit SHA1, if it happens to be available in the bitmap index, its bitmap will represent every single object that is reachable from the commit itself. The nth bit in the bitmap is the nth object in the packfile; if it's set to 1, the object is reachable. By using the bitmaps available in the index, this commit implements several new functions: - `prepare_bitmap_git` - `prepare_bitmap_walk` - `traverse_bitmap_commit_list` - `reuse_partial_packfile_from_bitmap` The `prepare_bitmap_walk` function tries to build a bitmap of all the objects that can be reached from the commit roots of a given `rev_info` struct by using the following algorithm: - If all the interesting commits for a revision walk are available in the index, the resulting reachability bitmap is the bitwise OR of all the individual bitmaps. - When the full set of WANTs is not available in the index, we perform a partial revision walk using the commits that don't have bitmaps as roots, and limiting the revision walk as soon as we reach a commit that has a corresponding bitmap. The earlier OR'ed bitmap with all the indexed commits can now be completed as this walk progresses, so the end result is the full reachability list. - For revision walks with a HAVEs set (a set of commits that are deemed uninteresting), first we perform the same method as for the WANTs, but using our HAVEs as roots, in order to obtain a full reachability bitmap of all the uninteresting commits. This bitmap then can be used to: a) limit the subsequent walk when building the WANTs bitmap b) finding the final set of interesting commits by performing an AND-NOT of the WANTs and the HAVEs. If `prepare_bitmap_walk` runs successfully, the resulting bitmap is stored and the equivalent of a `traverse_commit_list` call can be performed by using `traverse_bitmap_commit_list`; the bitmap version of this call yields the objects straight from the packfile index (without having to look them up or parse them) and hence is several orders of magnitude faster. As an extra optimization, when `prepare_bitmap_walk` succeeds, the `reuse_partial_packfile_from_bitmap` call can be attempted: it will find the amount of objects at the beginning of the on-disk packfile that can be reused as-is, and return an offset into the packfile. The source packfile can then be loaded and the bytes up to `offset` can be written directly to the result without having to consider the entires inside the packfile individually. If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files are available), the `rev_info` struct is left untouched, and can be used to perform a manual rev-walk using `traverse_commit_list`. Hence, this new set of functions are a generic API that allows to perform the equivalent of git rev-list --objects [roots...] [^uninteresting...] for any set of commits, even if they don't have specific bitmaps generated for them. In further patches, we'll use this bitmap traversal optimization to speed up the `pack-objects` and `rev-list` commands. Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-21 14:00:01 +00:00
#endif