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a8dd7e05b1
Back ine37d0b8730(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 sincee37d0b8730was 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>
34 lines
745 B
Bash
Executable file
34 lines
745 B
Bash
Executable file
#!/bin/sh
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test_description='Tests pack performance using bitmaps (rev index enabled)'
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. ./perf-lib.sh
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. "${TEST_DIRECTORY}/perf/lib-bitmap.sh"
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test_lookup_pack_bitmap () {
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test_expect_success 'start the test from scratch' '
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rm -rf * .git
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'
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test_perf_large_repo
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test_expect_success 'setup bitmap config' '
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git config pack.writebitmaps true
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'
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# we need to create the tag up front such that it is covered by the repack and
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# thus by generated bitmaps.
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test_expect_success 'create tags' '
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git tag --message="tag pointing to HEAD" perf-tag HEAD
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'
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test_perf "enable lookup table: $1" '
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git config pack.writeBitmapLookupTable '"$1"'
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'
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test_pack_bitmap
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}
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test_lookup_pack_bitmap false
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test_lookup_pack_bitmap true
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test_done
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