Back in e37d0b8730 (builtin/index-pack.c: write reverse indexes,
2021-01-25), Git learned how to read and write a pack's reverse index
from a file instead of in-memory.
A pack's reverse index is a mapping from pack position (that is, the
order that objects appear together in a ".pack") to their position in
lexical order (that is, the order that objects are listed in an ".idx"
file).
Reverse indexes are consulted often during pack-objects, as well as
during auxiliary operations that require mapping between pack offsets,
pack order, and index index.
They are useful in GitHub's infrastructure, where we have seen a
dramatic increase in performance when writing ".rev" files[1]. In
particular:
- an ~80% reduction in the time it takes to serve fetches on a popular
repository, Homebrew/homebrew-core.
- a ~60% reduction in the peak memory usage to serve fetches on that
same repository.
- a collective savings of ~35% in CPU time across all pack-objects
invocations serving fetches across all repositories in a single
datacenter.
Reverse indexes are also beneficial to end-users as well as forges. For
example, the time it takes to generate a pack containing the objects for
the 10 most recent commits in linux.git (representing a typical push) is
significantly faster when on-disk reverse indexes are available:
$ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~10 } >in
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null
Time (mean ± σ): 543.0 ms ± 20.3 ms [User: 616.2 ms, System: 58.8 ms]
Range (min … max): 521.0 ms … 577.9 ms 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null
Time (mean ± σ): 245.0 ms ± 11.4 ms [User: 335.6 ms, System: 31.3 ms]
Range (min … max): 226.0 ms … 259.6 ms 13 runs
Summary
'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran
2.22 ± 0.13 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
The same is true of writing a pack containing the objects for the 30
most-recent commits:
$ { git rev-parse HEAD && printf '^' && git rev-parse HEAD~30 } >in
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
Benchmark 1: git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null
Time (mean ± σ): 866.5 ms ± 16.2 ms [User: 1414.5 ms, System: 97.0 ms]
Range (min … max): 839.3 ms … 886.9 ms 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null
Time (mean ± σ): 581.6 ms ± 10.2 ms [User: 1181.7 ms, System: 62.6 ms]
Range (min … max): 567.5 ms … 599.3 ms 10 runs
Summary
'git.compile -c pack.readReverseIndex=true pack-objects --delta-base-offset --revs --stdout <in >/dev/null' ran
1.49 ± 0.04 times faster than 'git.compile -c pack.readReverseIndex=false pack-objects --delta-base-offset --revs --stdout <in >/dev/null'
...and savings on trivial operations like computing the on-disk size of
a single (packed) object are even more dramatic:
$ git rev-parse HEAD >in
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" <in'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in
Time (mean ± σ): 305.8 ms ± 11.4 ms [User: 264.2 ms, System: 41.4 ms]
Range (min … max): 290.3 ms … 331.1 ms 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in
Time (mean ± σ): 4.0 ms ± 0.3 ms [User: 1.7 ms, System: 2.3 ms]
Range (min … max): 1.6 ms … 4.6 ms 1155 runs
Summary
'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran
76.96 ± 6.25 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in'
In the more than two years since e37d0b8730 was merged, Git's
implementation of on-disk reverse indexes has been thoroughly tested,
both from users enabling `pack.writeReverseIndexes`, and from GitHub's
deployment of the feature. The latter has been running without incident
for more than two years.
This patch changes Git's behavior to write on-disk reverse indexes by
default when indexing a pack, which should make the above operations
faster for everybody's Git installation after a repack.
(The previous commit explains some potential drawbacks of using on-disk
reverse indexes in certain limited circumstances, that essentially boil
down to a trade-off between time to generate, and time to access. For
those limited cases, the `pack.readReverseIndex` escape hatch can be
used).
[1]: https://github.blog/2021-04-29-scaling-monorepo-maintenance/#reverse-indexes
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add performance tests to verify the performance of lookup table.
`p5310-pack-bitmaps.sh` contain tests with and without lookup table.
`p5312-pack-bitmaps-revs.sh` contain same tests with and without
lookup table but with `pack.writeReverseIndex` enabled.
Lookup table makes Git run faster in most of the cases. Below is the
result of `t/perf/p5310-pack-bitmaps.sh`.`perf/p5326-multi-pack-bitmaps.sh`
gives similar result. The repository used in the test is linux kernel.
Test this tree
-----------------------------------------------------------------------
5310.4: enable lookup table: false 0.01(0.00+0.00)
5310.5: repack to disk 320.89(230.20+23.45)
5310.6: simulated clone 14.04(5.78+1.79)
5310.7: simulated fetch 1.95(3.05+0.20)
5310.8: pack to file (bitmap) 44.73(20.55+7.45)
5310.9: rev-list (commits) 0.78(0.46+0.10)
5310.10: rev-list (objects) 4.07(3.97+0.08)
5310.11: rev-list with tag negated via --not 0.06(0.02+0.03)
--all (objects)
5310.12: rev-list with negative tag (objects) 0.21(0.15+0.05)
5310.13: rev-list count with blob:none 0.24(0.17+0.06)
5310.14: rev-list count with blob:limit=1k 7.07(5.92+0.48)
5310.15: rev-list count with tree:0 0.25(0.17+0.07)
5310.16: simulated partial clone 5.67(3.28+0.64)
5310.18: clone (partial bitmap) 16.05(8.34+1.86)
5310.19: pack to file (partial bitmap) 59.76(27.22+7.43)
5310.20: rev-list with tree filter (partial bitmap) 0.90(0.18+0.16)
5310.24: enable lookup table: true 0.01(0.00+0.00)
5310.25: repack to disk 319.73(229.30+23.01)
5310.26: simulated clone 13.69(5.72+1.78)
5310.27: simulated fetch 1.84(3.02+0.16)
5310.28: pack to file (bitmap) 45.63(20.67+7.50)
5310.29: rev-list (commits) 0.56(0.39+0.8)
5310.30: rev-list (objects) 3.77(3.74+0.08)
5310.31: rev-list with tag negated via --not 0.05(0.02+0.03)
--all (objects)
5310.32: rev-list with negative tag (objects) 0.21(0.15+0.05)
5310.33: rev-list count with blob:none 0.23(0.17+0.05)
5310.34: rev-list count with blob:limit=1k 6.65(5.72+0.40)
5310.35: rev-list count with tree:0 0.23(0.16+0.06)
5310.36: simulated partial clone 5.57(3.26+0.59)
5310.38: clone (partial bitmap) 15.89(8.39+1.84)
5310.39: pack to file (partial bitmap) 58.32(27.55+7.47)
5310.40: rev-list with tree filter (partial bitmap) 0.73(0.18+0.15)
Test 4-15 are tested without using lookup table. Same tests are
repeated in 16-30 (using lookup table).
Mentored-by: Taylor Blau <me@ttaylorr.com>
Co-Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Taylor Blau