When constructing a new pack `git multi-pack-index repack` provides a
list of objects which is the union of objects in all MIDX'd packs which
were "included" in the repack.
Though correct, this typically yields a poorly structured pack, since
providing the objects list over stdin does not give pack-objects a
chance to discover the namehash values for each object, leading to
sub-optimal delta selection.
We can use `--stdin-packs` instead, which has a couple of benefits:
- it does a supplemental walk over objects in the supplied list of
packs to discover their namehash, leading to higher-quality delta
selection
- it requires us to list far less data over stdin; instead of listing
each object in the resulting pack, we need only list the
constituent packs from which those objects were selected in the MIDX
Of course, this comes at a slight cost: though we save time on listing
packs versus objects over stdin[^1] (around ~650 milliseconds), we add a
non-trivial amount of time walking over the given objects in order to
find better deltas.
In general, this is likely to more closely match the user's expectations
(i.e. that packs generated via `git multi-pack-index repack` are written
with high-quality deltas). But if not, we can always introduce a new
option in pack-objects to disable the supplemental object walk, which
would yield a pure CPU-time savings, at the cost of the on-disk size of
the resulting pack.
[^1]: In a patched version of Git that doesn't perform the supplemental
object walk in `pack-objects --stdin-packs`, we save around ~650ms
(from 5.968 to 5.325 seconds) when running `git multi-pack-index
repack --batch-size=0` on git.git with all objects packed, and all
packs in a MIDX.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In both fill_included_packs_all() and fill_included_packs_batch(), we
accumulate a list of packs whose contents we want to repack together,
and then use that information to feed a list of objects as input to
pack-objects.
In both cases, the `fill_included_packs_` functions keep track of how
many packs they want to repack together, and only execute pack-objects
if there are at least two packs that need repacking.
Having both of these functions keep track of this information themselves
is not strictly necessary, since they also log which packs to repack via
the `include_pack` array, so we can simply count the non-zero entries in
that array after either function is done executing, reducing the overall
amount of code necessary.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When performing a 'git multi-pack-index repack', the MIDX machinery
tries to aggregate MIDX'd packs together either to (a) fill the given
`--batch-size` argument, or (b) combine all packs together.
In either case (using the `midx-write.c::fill_included_packs_batch()` or
`midx-write.c::fill_included_packs_all()` function, respectively), we
evaluate whether or not we want to repack each MIDX'd pack, according to
whether or it is loadable, kept, cruft, or non-empty.
Between the two `fill_included_packs_` callers, they both care about the
same conditions, except for `fill_included_packs_batch()` which also
cares that the pack is non-empty.
We could extract two functions (say, `want_included_pack()` and a
`_nonempty()` variant), but this is not necessary. For the case in
`fill_included_packs_all()` which does not check the pack size, we add
all of the pack's objects assuming that the pack meets all other
criteria. But if the pack is empty in the first place, we add all of its
zero objects, so whether or not we "accept" or "reject" it in the first
place is irrelevant.
This change improves the readability in both `fill_included_packs_`
functions.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Introduce a new midx-write.c source file, which holds all of the
functionality from the MIDX sub-system related to writing new MIDX files.
Similar to the relationship between "pack-bitmap.c" and
"pack-bitmap-write.c", this source file will hold code that is specific
to writing MIDX files as opposed to reading them (the latter will remain
in midx.c).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
