[PATCH v2 5/7] pack-revindex: introduce `pack.readReverseIndex`
- From
Taylor Blau <me@ttaylorr.com>
- Date
- Apr 12, 2023, 22:20 UTC
- Message-ID
- <a62fc3e4ec1aca773b6c3e47828db235df53c747.1681338013.git.me@ttaylorr.com>
- In-Reply-To
- <cover.1681338013.git.me@ttaylorr.com>
Since 1615c567b8 (Documentation/config/pack.txt: advertise 'pack.writeReverseIndex', 2021-01-25), we have had the `pack.writeReverseIndex` configuration option, which tells Git whether or not it is allowed to write a ".rev" file when indexing a pack.
Introduce a complementary configuration knob, `pack.readReverseIndex` to control whether or not Git will read any ".rev" file(s) that may be available on disk.
This option is useful for debugging, as well as disabling the effect of ".rev" files in certain instances.
This is useful because of the trade-off[^1] between the time it takes to generate a reverse index (slow from scratch, fast when reading an existing ".rev" file), and the time it takes to access a record (the opposite).
For example, even though it is faster to use the on-disk reverse index when computing the on-disk size of a packed object, it is slower to enumerate the same value for all objects.
Here are a couple of examples from linux.git. When computing the above for a single object, using the on-disk reverse index is significantly faster:
$ 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): 302.5 ms ± 12.5 ms [User: 258.7 ms, System: 43.6 ms]
Range (min … max): 291.1 ms … 328.1 ms 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in
Time (mean ± σ): 3.9 ms ± 0.3 ms [User: 1.6 ms, System: 2.4 ms]
Range (min … max): 2.0 ms … 4.4 ms 801 runs Summary
'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran
77.29 ± 7.14 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in', but when instead trying to compute the on-disk object size for all objects in the repository, using the ".rev" file is a disadvantage over creating the reverse index from scratch:
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 8.258 s ± 0.035 s [User: 7.949 s, System: 0.308 s]
Range (min … max): 8.199 s … 8.293 s 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 16.976 s ± 0.107 s [User: 16.706 s, System: 0.268 s]
Range (min … max): 16.839 s … 17.105 s 10 runs Summary
'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects' ran
2.06 ± 0.02 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'Luckily, the results when running `git cat-file` with `--unordered` are closer together:
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 5.066 s ± 0.105 s [User: 4.792 s, System: 0.274 s]
Range (min … max): 4.943 s … 5.220 s 10 runs Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 6.193 s ± 0.069 s [User: 5.937 s, System: 0.255 s]
Range (min … max): 6.145 s … 6.356 s 10 runs Summary
'git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects' ran
1.22 ± 0.03 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'Because the equilibrium point between these two is highly machine- and repository-dependent, allow users to configure whether or not they will read any ".rev" file(s) with this configuration knob.
[^1]: Generating a reverse index in memory takes O(N) time (where N is the number of objects in the repository), since we use a radix sort. Reading an entry from an on-disk ".rev" file is slower since each operation is bound by disk I/O instead of memory I/O.
In order to compute the on-disk size of a packed object, we need to find the offset of our object, and the adjacent object (the on-disk size difference of these two). Finding the first offset requires a binary search. Finding the latter involves a single .rev lookup.
Signed-off-by: Taylor Blau <me@ttaylorr.com> --- Documentation/config/pack.txt | 6 ++++++ pack-revindex.c | 5 ++++- repo-settings.c | 1 + repository.h | 1 + t/t5325-reverse-index.sh | 11 +++++++++++ 5 files changed, 23 insertions(+), 1 deletion(-)
diff --git a/Documentation/config/pack.txt b/Documentation/config/pack.txt index 53093d99969..7db7fed4667 100644 --- a/Documentation/config/pack.txt +++ b/Documentation/config/pack.txt @@ -171,6 +171,12 @@ pack.writeBitmapLookupTable:: beneficial in repositories that have relatively large bitmap indexes. Defaults to false. +pack.readReverseIndex:: + When true, git will read any .rev file(s) that may be available + (see: linkgit:gitformat-pack[5]). When false, the reverse index + will be generated from scratch and stored in memory. Defaults to + true. + pack.writeReverseIndex:: When true, git will write a corresponding .rev file (see: linkgit:gitformat-pack[5]) diff --git a/pack-revindex.c b/pack-revindex.c index 44e1b3fed95..29f5358b256 100644 --- a/pack-revindex.c +++ b/pack-revindex.c @@ -291,7 +291,10 @@ int load_pack_revindex(struct repository *r, struct packed_git *p) if (p->revindex || p->revindex_data) return 0; - if (!load_pack_revindex_from_disk(p)) + prepare_repo_settings(r); + + if (r->settings.pack_read_reverse_index && + !load_pack_revindex_from_disk(p)) return 0; else if (!create_pack_revindex_in_memory(p)) return 0; diff --git a/repo-settings.c b/repo-settings.c index 0a6c0b381fe..bdd7640ab0b 100644 --- a/repo-settings.c +++ b/repo-settings.c @@ -63,6 +63,7 @@ void prepare_repo_settings(struct repository *r) repo_cfg_bool(r, "core.multipackindex", &r->settings.core_multi_pack_index, 1); repo_cfg_bool(r, "index.sparse", &r->settings.sparse_index, 0); repo_cfg_bool(r, "index.skiphash", &r->settings.index_skip_hash, r->settings.index_skip_hash); + repo_cfg_bool(r, "pack.readreverseindex", &r->settings.pack_read_reverse_index, 1); /* * The GIT_TEST_MULTI_PACK_INDEX variable is special in that diff --git a/repository.h b/repository.h index 15a8afc5fb5..ed73e799b61 100644 --- a/repository.h +++ b/repository.h @@ -37,6 +37,7 @@ struct repo_settings { int fetch_write_commit_graph; int command_requires_full_index; int sparse_index; + int pack_read_reverse_index; struct fsmonitor_settings *fsmonitor; /* lazily loaded */ diff --git a/t/t5325-reverse-index.sh b/t/t5325-reverse-index.sh index 48c14d85764..66171c1d67b 100755 --- a/t/t5325-reverse-index.sh +++ b/t/t5325-reverse-index.sh @@ -96,6 +96,17 @@ test_expect_success 'reverse index is not generated when available on disk' ' --batch-check="%(objectsize:disk)" <tip ' +test_expect_success 'reverse index is ignored when pack.readReverseIndex is false' ' + test_index_pack true && + test_path_is_file $rev && + + test_config pack.readReverseIndex false && + + git rev-parse HEAD >tip && + GIT_TEST_REV_INDEX_DIE_ON_DISK=1 git cat-file \ + --batch-check="%(objectsize:disk)" <tip +' + test_expect_success 'revindex in-memory vs on-disk' ' git init repo && test_when_finished "rm -fr repo" &&
-- 2.40.0.323.gedff6a80c63