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When seeking a reftable record in a block we need to position the
iterator _before_ the sought-after record so that the next call to
`block_iter_next()` would yield that record. To achieve this, the loop
that performs the linear seeks to restore the previous position once it
has found the record.
This is done by advancing two `block_iter`s: one to check whether the
next record is our sought-after record, and one that we update after
every iteration. This of course involves quite a lot of copying and also
leads to needless memory allocations.
Refactor the code to get rid of the `next` iterator and the copying this
involves. Instead, we can restore the previous offset such that the call
to `next` will return the correct record.
Next to being simpler conceptually this also leads to a nice speedup.
The following benchmark parser 10k refs out of 100k existing refs via
`git-rev-list --no-walk`:
Benchmark 1: rev-list: print many refs (HEAD~)
Time (mean ± σ): 170.2 ms ± 1.7 ms [User: 86.1 ms, System: 83.6 ms]
Range (min … max): 166.4 ms … 180.3 ms 500 runs
Benchmark 2: rev-list: print many refs (HEAD~)
Time (mean ± σ): 161.6 ms ± 1.6 ms [User: 78.1 ms, System: 83.0 ms]
Range (min … max): 158.4 ms … 172.3 ms 500 runs
Summary
rev-list: print many refs (HEAD) ran
1.05 ± 0.01 times faster than rev-list: print many refs (HEAD~)
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
143 lines
3.8 KiB
C
143 lines
3.8 KiB
C
/*
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Copyright 2020 Google LLC
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Use of this source code is governed by a BSD-style
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license that can be found in the LICENSE file or at
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https://developers.google.com/open-source/licenses/bsd
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*/
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#ifndef BLOCK_H
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#define BLOCK_H
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#include "basics.h"
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#include "record.h"
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#include "reftable-blocksource.h"
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/*
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* Writes reftable blocks. The block_writer is reused across blocks to minimize
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* allocation overhead.
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*/
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struct block_writer {
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uint8_t *buf;
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uint32_t block_size;
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/* Offset of the global header. Nonzero in the first block only. */
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uint32_t header_off;
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/* How often to restart keys. */
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int restart_interval;
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int hash_size;
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/* Offset of next uint8_t to write. */
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uint32_t next;
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uint32_t *restarts;
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uint32_t restart_len;
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uint32_t restart_cap;
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struct strbuf last_key;
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int entries;
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};
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/*
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* initializes the blockwriter to write `typ` entries, using `buf` as temporary
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* storage. `buf` is not owned by the block_writer. */
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void block_writer_init(struct block_writer *bw, uint8_t typ, uint8_t *buf,
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uint32_t block_size, uint32_t header_off, int hash_size);
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/* returns the block type (eg. 'r' for ref records. */
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uint8_t block_writer_type(struct block_writer *bw);
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/* appends the record, or -1 if it doesn't fit. */
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int block_writer_add(struct block_writer *w, struct reftable_record *rec);
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/* appends the key restarts, and compress the block if necessary. */
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int block_writer_finish(struct block_writer *w);
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/* clears out internally allocated block_writer members. */
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void block_writer_release(struct block_writer *bw);
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struct z_stream;
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/* Read a block. */
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struct block_reader {
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/* offset of the block header; nonzero for the first block in a
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* reftable. */
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uint32_t header_off;
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/* the memory block */
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struct reftable_block block;
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int hash_size;
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/* Uncompressed data for log entries. */
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z_stream *zstream;
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unsigned char *uncompressed_data;
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size_t uncompressed_cap;
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/* size of the data, excluding restart data. */
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uint32_t block_len;
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uint8_t *restart_bytes;
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uint16_t restart_count;
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/* size of the data in the file. For log blocks, this is the compressed
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* size. */
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uint32_t full_block_size;
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};
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/* initializes a block reader. */
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int block_reader_init(struct block_reader *br, struct reftable_block *bl,
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uint32_t header_off, uint32_t table_block_size,
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int hash_size);
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void block_reader_release(struct block_reader *br);
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/* Returns the block type (eg. 'r' for refs) */
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uint8_t block_reader_type(const struct block_reader *r);
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/* Decodes the first key in the block */
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int block_reader_first_key(const struct block_reader *br, struct strbuf *key);
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/* Iterate over entries in a block */
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struct block_iter {
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/* offset within the block of the next entry to read. */
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uint32_t next_off;
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const unsigned char *block;
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size_t block_len;
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int hash_size;
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/* key for last entry we read. */
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struct strbuf last_key;
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struct strbuf scratch;
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};
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#define BLOCK_ITER_INIT { \
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.last_key = STRBUF_INIT, \
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.scratch = STRBUF_INIT, \
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}
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/* Position `it` at start of the block */
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void block_iter_seek_start(struct block_iter *it, const struct block_reader *br);
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/* Position `it` to the `want` key in the block */
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int block_iter_seek_key(struct block_iter *it, const struct block_reader *br,
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struct strbuf *want);
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/* return < 0 for error, 0 for OK, > 0 for EOF. */
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int block_iter_next(struct block_iter *it, struct reftable_record *rec);
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/* Reset the block iterator to pristine state without releasing its memory. */
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void block_iter_reset(struct block_iter *it);
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/* deallocate memory for `it`. The block reader and its block is left intact. */
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void block_iter_close(struct block_iter *it);
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/* size of file header, depending on format version */
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int header_size(int version);
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/* size of file footer, depending on format version */
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int footer_size(int version);
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/* returns a block to its source. */
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void reftable_block_done(struct reftable_block *ret);
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#endif
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