From ebcf26b12b38f080dbbdf963b921ea5e9bcec6ae Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Thu, 28 Nov 2024 12:23:09 -0500 Subject: [PATCH] pack-objects: add third name hash version The '--name-hash-version=' option in 'git pack-objects' was introduced to allow for specifying an alternative name hash function when organizing objects for delta compression. The pack_name_hash_v2() function was designed to break some collisions while also preserving some amount of locality for cross-path deltas. However, in some repositories, that effort to preserve locality results in enough collisions that it causes issues with full repacks. Create a third name hash function and extend the '--name-hash-version' option in 'git pack-objects' and 'git repack' to understand it. This hash version abandons all efforts for locality and focuses on creating a somewhat uniformly-distributed hash function to minimize collisions. We can observe the effect of this collision avoidance in a large internal monorepo that suffered from collisions in the previous versions. The updates to p5314-name-hash.sh show these results: Test this tree -------------------------------------------------- 5314.1: paths at head 227.3K 5314.2: distinct hash value: v1 72.3K 5314.3: maximum multiplicity: v1 14.4K 5314.4: distinct hash value: v2 166.5K 5314.5: maximum multiplicity: v2 138 5314.6: distinct hash value: v3 227.3K 5314.7: maximum multiplicity: v3 2 These results demonstrate that of the 227,000+ paths, nearly all of them find distinct hash values. The maximum multiplicity is 2, improved from 138 in the v2 hash function. The v2 hash function also had only 166K distinct values, so it had a wide spread of collisions. A more modest improvement is available in the open source fluentui repo [1] with these results: Test this tree -------------------------------------------------- 5314.1: paths at head 19.5K 5314.2: distinct hash value: v1 8.2K 5314.3: maximum multiplicity: v1 279 5314.4: distinct hash value: v2 17.8K 5314.5: maximum multiplicity: v2 44 5314.6: distinct hash value: v3 19.5K 5314.7: maximum multiplicity: v3 1 [1] https://github.com/microsoft/fluentui However, it is important to demonstrate the effectiveness of this function in the context of compressing a repository. We can use p5313-pack-objects.sh to measure these changes. I will use a simplified table summarizing the output of that performance test. | Test | V1 Time | V2 Time | V3 Time | V1 Size | V2 Size | V3 Size | |-----------|---------|---------|---------|---------|---------|---------| | Thin Pack | 0.37 s | 0.12 s | 0.07 s | 1.2 M | 22.0 K | 20.4 K | | Big Pack | 2.04 s | 2.80 s | 1.40 s | 20.4 M | 25.9 M | 19.2 M | | Shallow | 1.41 s | 1.77 s | 1.27 s | 34.4 M | 33.7 M | 34.8 M | | Repack | 95.70 s | 33.68 s | 20.88 s | 439.3 M | 160.5 M | 169.1 M | Here, there are some performance improvements on a time basis, and the thin and big packs are somewhat smaller in v3. The shallow and repacked packs are somewhat bigger, though, compared to v2. Two repositories that have very few collisions in the v1 name hash are the Git and Linux repositories. Here are their stats for p5313: Git: | Test | V1 Time | V2 Time | V3 Time | V1 Size | V2 Size | V3 Size | |-----------|---------|---------|---------|---------|---------|---------| | Thin Pack | 0.02 s | 0.02 s | 0.02 s | 1.1 K | 1.1 K | 15.3 K | | Big Pack | 1.69 s | 1.95 s | 1.67 s | 13.5 M | 14.5 M | 14.9 M | | Shallow | 1.26 s | 1.29 s | 1.16 s | 12.0 M | 12.2 M | 12.5 M | | Repack | 29.51 s | 29.01 s | 29.08 s | 237.7 M | 238.2 M | 237.7 M | Linux: | Test | V1 Time | V2 Time | V3 Time | V1 Size | V2 Size | V3 Size | |-----------|----------|----------|----------|---------|---------|---------| | Thin Pack | 0.17 s | 0.07 s | 0.07 s | 4.6 K | 4.6 K | 6.8 K | | Big Pack | 17.88 s | 12.35 s | 12.14 s | 201.1 M | 149.1 M | 160.4 M | | Shallow | 11.05 s | 22.94 s | 22.16 s | 269.2 M | 273.8 M | 271.8 M | | Repack | 727.39 s | 566.95 s | 539.33 s | 2.5 G | 2.5 G | 2.6 G | These repositories make good use of the cross-path deltas that come about from the v1 name hash function, so they already had mixed results with the v2 function. The v3 function is generally worse for these repositories. An internal Javascript-based repository with name hash collisions similar to the fluentui repo has these results: | Test | V1 Time | V2 Time | V3 Time | V1 Size | V2 Size | V3 Size | |-----------|-----------|----------|----------|---------|---------|---------| | Thin Pack | 8.28 s | 7.28 s | 0.04 s | 16.8 K | 16.8 K | 3.2 K | | Big Pack | 12.81 s | 11.66 s | 2.52 s | 29.1 M | 29.1 M | 30.6 M | | Shallow | 4.86 s | 4.06 s | 3.77 s | 42.5 M | 44.1 M | 45.7 M | | Repack | 3126.50 s | 496.33 s | 306.86 s | 6.2 G | 855.6 M | 838.2 M | This repository is also listed as "Repo B" in the repacking size table below, along with other Javascript repos that have many name hash collisions with the v1 name hash: | Repo | V1 Size | V2 Size | V3 Size | |----------|-----------|---------|---------| | fluentui | 440 M | 161 M | 170 M | | Repo B | 6,248 M | 856 M | 840 M | | Repo C | 37,278 M | 6,921 M | 6,755 M | | Repo D | 131,204 M | 7,463 M | 7,124 M | While the fluentui repo had an increase in size using the v3 name hash, the others had modest improvements over the v2 name hash. But those modest improvements are dwarfed by the difference from v1 to v2, so it is unlikely that the regression seen in the other scenarios (packfiles that are not from full repacks) will be worth using v3 over v2. That is, unless there are enough collisions even with v2 that the full repack scenario has larger improvements than these. Signed-off-by: Derrick Stolee --- Documentation/git-pack-objects.txt | 9 +++++++++ Documentation/git-repack.txt | 9 +++++++++ builtin/pack-objects.c | 5 ++++- pack-objects.h | 26 ++++++++++++++++++++++++++ t/helper/test-name-hash.c | 1 + t/perf/p5313-pack-objects.sh | 2 +- t/perf/p5314-name-hash.sh | 2 +- t/t5300-pack-object.sh | 4 ++-- t/t5310-pack-bitmaps.sh | 14 +++++++------- 9 files changed, 60 insertions(+), 12 deletions(-) diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt index 7f69ae4855f6ce..9fe25c5341576c 100644 --- a/Documentation/git-pack-objects.txt +++ b/Documentation/git-pack-objects.txt @@ -374,6 +374,15 @@ breaking most of the collisions from a similarly-named file appearing in many different directories. At the moment, this version is not allowed when writing reachability bitmap files with `--write-bitmap-index` and it will be automatically changed to version `1`. ++ +The name hash version `3` abandons the locality features of versions `1` +and `2` in favor of minimizing collisions. The goal here is to separate +objects by their full path and abandon hope for cross-path delta +compression. For this reason, this option is preferred for repacking large +repositories with many versions and many name hash collisions when using +the first two versions. At the moment, this version is not allowed when +writing reachability bitmap files with `--write-bitmap-index` and it will +be automatically changed to version `1`. DELTA ISLANDS diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt index ea69fbe1891c90..bc74d9333f012b 100644 --- a/Documentation/git-repack.txt +++ b/Documentation/git-repack.txt @@ -279,6 +279,15 @@ breaking most of the collisions from a similarly-named file appearing in many different directories. At the moment, this version is not allowed when writing reachability bitmap files with `--write-bitmap-index` and it will be automatically changed to version `1`. ++ +The name hash version `3` abandons the locality features of versions `1` +and `2` in favor of minimizing collisions. The goal here is to separate +objects by their full path and abandon hope for cross-path delta +compression. For this reason, this option is preferred for repacking large +repositories with many versions and many name hash collisions when using +the first two versions. At the moment, this version is not allowed when +writing reachability bitmap files with `--write-bitmap-index` and it will +be automatically changed to version `1`. CONFIGURATION diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c index e2f6431d614fb1..e8cbcb8773bc5b 100644 --- a/builtin/pack-objects.c +++ b/builtin/pack-objects.c @@ -270,7 +270,7 @@ static int name_hash_version = -1; static void validate_name_hash_version(void) { - if (name_hash_version < 1 || name_hash_version > 2) + if (name_hash_version < 1 || name_hash_version > 3) die(_("invalid --name-hash-version option: %d"), name_hash_version); } @@ -284,6 +284,9 @@ static inline uint32_t pack_name_hash_fn(const char *name) case 2: return pack_name_hash_v2(name); + case 3: + return pack_name_hash_v3(name); + default: BUG("invalid name-hash version: %d", name_hash_version); } diff --git a/pack-objects.h b/pack-objects.h index 15be8368d21d8c..8c0840983e1a24 100644 --- a/pack-objects.h +++ b/pack-objects.h @@ -235,6 +235,32 @@ static inline uint32_t pack_name_hash_v2(const char *name) return (base >> 6) ^ hash; } +static inline uint32_t pack_name_hash_v3(const char *name) +{ + /* + * This 'bigp' value is a large prime, at least 25% of the max + * value of an uint32_t. Multiplying by this value (modulo 2^32) + * causes the 32 bits to change pseudo-randomly. + */ + const uint32_t bigp = 1234572167U; + uint32_t c, hash = bigp; + + if (!name) + return 0; + + /* + * Do the simplest thing that will resemble pseudo-randomness: add + * random multiples of a large prime number with a binary shift. + * The goal is not to be cryptographic, but to be generally + * uniformly distributed. + */ + while ((c = *name++) != 0) { + hash += c * bigp; + hash = (hash >> 5) | (hash << 27); + } + return hash; +} + static inline enum object_type oe_type(const struct object_entry *e) { return e->type_valid ? e->type_ : OBJ_BAD; diff --git a/t/helper/test-name-hash.c b/t/helper/test-name-hash.c index 5b40236202064e..1f71a41e2a836f 100644 --- a/t/helper/test-name-hash.c +++ b/t/helper/test-name-hash.c @@ -15,6 +15,7 @@ int cmd__name_hash(int argc UNUSED, const char **argv UNUSED) while (!strbuf_getline(&line, stdin)) { printf("%10u ", pack_name_hash(line.buf)); printf("%10u ", pack_name_hash_v2(line.buf)); + printf("%10u ", pack_name_hash_v3(line.buf)); printf("%s\n", line.buf); } diff --git a/t/perf/p5313-pack-objects.sh b/t/perf/p5313-pack-objects.sh index be5229a0ecdcf5..493872e656d295 100755 --- a/t/perf/p5313-pack-objects.sh +++ b/t/perf/p5313-pack-objects.sh @@ -25,7 +25,7 @@ test_expect_success 'create rev input' ' EOF ' -for version in 1 2 +for version in 1 2 3 do export version diff --git a/t/perf/p5314-name-hash.sh b/t/perf/p5314-name-hash.sh index 4ef0ba771143d8..e58a218d1ae4e9 100755 --- a/t/perf/p5314-name-hash.sh +++ b/t/perf/p5314-name-hash.sh @@ -14,7 +14,7 @@ test_size 'paths at head' ' test-tool name-hash name-hashes ' -for version in 1 2 +for version in 1 2 3 do test_size "distinct hash value: v$version" ' awk "{ print \$$version; }" err && test_grep "invalid --name-hash-version option" err || return 1 diff --git a/t/t5310-pack-bitmaps.sh b/t/t5310-pack-bitmaps.sh index 871ce01401a11e..2bf75e2a5d0e6a 100755 --- a/t/t5310-pack-bitmaps.sh +++ b/t/t5310-pack-bitmaps.sh @@ -45,13 +45,13 @@ test_expect_success 'name-hash value stability' ' test-tool name-hash out && cat >expect <<-\EOF && - 2582249472 1763573760 first - 2289942528 1188134912 second - 2300837888 1130758144 third - 2544516325 3963087891 a/one-long-enough-for-collisions - 2544516325 4013419539 b/two-long-enough-for-collisions - 1420111091 1709547268 many/parts/to/this/path/enough/to/collide/in/v2 - 1420111091 1709547268 enough/parts/to/this/path/enough/to/collide/in/v2 + 2582249472 1763573760 3109209818 first + 2289942528 1188134912 3781118409 second + 2300837888 1130758144 3028707182 third + 2544516325 3963087891 3586976147 a/one-long-enough-for-collisions + 2544516325 4013419539 1701624798 b/two-long-enough-for-collisions + 1420111091 1709547268 2676129939 many/parts/to/this/path/enough/to/collide/in/v2 + 1420111091 1709547268 2740459187 enough/parts/to/this/path/enough/to/collide/in/v2 EOF test_cmp expect out