From: Derrick Stolee <stolee@xxxxxxxxx> Since 'git pack-objects' supports a --path-walk option, allow passing it through in 'git repack'. This presents interesting testing opportunities for comparing the different repacking strategies against each other. Add the --path-walk option to the performance tests in p5313. For the microsoft/fluentui repo [1] checked out at a specific commit [2], the --path-walk tests in p5313 look like this: Test this tree ------------------------------------------------------------------------- 5313.18: thin pack with --path-walk 0.08(0.06+0.02) 5313.19: thin pack size with --path-walk 18.4K 5313.20: big pack with --path-walk 2.10(7.80+0.26) 5313.21: big pack size with --path-walk 19.8M 5313.22: shallow fetch pack with --path-walk 1.62(3.38+0.17) 5313.23: shallow pack size with --path-walk 33.6M 5313.24: repack with --path-walk 81.29(96.08+0.71) 5313.25: repack size with --path-walk 142.5M [1] https://github.com/microsoft/fluentui [2] e70848ebac1cd720875bccaa3026f4a9ed700e08 Along with the earlier tests in p5313, I'll instead reformat the comparison as follows: Repack Method Pack Size Time --------------------------------------- Hash v1 439.4M 87.24s Hash v2 161.7M 21.51s Path Walk 142.5M 81.29s There are a few things to notice here: 1. The benefits of --name-hash-version=2 over --name-hash-version=1 are significant, but --path-walk still compresses better than that option. 2. The --path-walk command is still using --name-hash-version=1 for the second pass of delta computation, using the increased name hash collisions as a potential method for opportunistic compression on top of the path-focused compression. 3. The --path-walk algorithm is currently sequential and does not use multiple threads for delta compression. Threading will be implemented in a future change so the computation time will improve to better compete in this metric. There are small benefits in size for my copy of the Git repository: Repack Method Pack Size Time --------------------------------------- Hash v1 248.8M 30.44s Hash v2 249.0M 30.15s Path Walk 213.2M 142.50s As well as in the nodejs/node repository [3]: Repack Method Pack Size Time --------------------------------------- Hash v1 739.9M 71.18s Hash v2 764.6M 67.82s Path Walk 698.1M 208.10s [3] https://github.com/nodejs/node This benefit also repeats in my copy of the Linux kernel repository: Repack Method Pack Size Time --------------------------------------- Hash v1 2.5G 554.41s Hash v2 2.5G 549.62s Path Walk 2.2G 1562.36s It is important to see that even when the repository shape does not have many name-hash collisions, there is a slight space boost to be found using this method. As this repacking strategy was released in Git for Windows 2.47.0, some users have reported cases where the --path-walk compression is slightly worse than the --name-hash-version=2 option. In those cases, it may be beneficial to combine the two options. However, there has not been a released version of Git that has both options and I don't have access to these repos for testing. Signed-off-by: Derrick Stolee <stolee@xxxxxxxxx> --- Documentation/git-repack.adoc | 14 +++++++++++++- builtin/repack.c | 7 ++++++- t/perf/p5313-pack-objects.sh | 18 ++++++++---------- 3 files changed, 27 insertions(+), 12 deletions(-) diff --git a/Documentation/git-repack.adoc b/Documentation/git-repack.adoc index 5852a5c9736..2199eb3b94f 100644 --- a/Documentation/git-repack.adoc +++ b/Documentation/git-repack.adoc @@ -11,7 +11,7 @@ SYNOPSIS [verse] 'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m] [--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>] - [--write-midx] [--name-hash-version=<n>] + [--write-midx] [--name-hash-version=<n>] [--path-walk] DESCRIPTION ----------- @@ -255,6 +255,18 @@ linkgit:git-multi-pack-index[1]). Provide this argument to the underlying `git pack-objects` process. See linkgit:git-pack-objects[1] for full details. +--path-walk:: + This option passes the `--path-walk` option to the underlying + `git pack-options` process (see linkgit:git-pack-objects[1]). + By default, `git pack-objects` walks objects in an order that + presents trees and blobs in an order unrelated to the path they + appear relative to a commit's root tree. The `--path-walk` option + enables a different walking algorithm that organizes trees and + blobs by path. This has the potential to improve delta compression + especially in the presence of filenames that cause collisions in + Git's default name-hash algorithm. Due to changing how the objects + are walked, this option is not compatible with `--delta-islands` + or `--filter`. CONFIGURATION ------------- diff --git a/builtin/repack.c b/builtin/repack.c index 75e3752353a..d7f798280c0 100644 --- a/builtin/repack.c +++ b/builtin/repack.c @@ -43,7 +43,7 @@ static char *packdir, *packtmp_name, *packtmp; static const char *const git_repack_usage[] = { N_("git repack [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m]\n" "[--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]\n" - "[--write-midx] [--name-hash-version=<n>]"), + "[--write-midx] [--name-hash-version=<n>] [--path-walk]"), NULL }; @@ -63,6 +63,7 @@ struct pack_objects_args { int quiet; int local; int name_hash_version; + int path_walk; struct list_objects_filter_options filter_options; }; @@ -313,6 +314,8 @@ static void prepare_pack_objects(struct child_process *cmd, strvec_pushf(&cmd->args, "--no-reuse-object"); if (args->name_hash_version) strvec_pushf(&cmd->args, "--name-hash-version=%d", args->name_hash_version); + if (args->path_walk) + strvec_pushf(&cmd->args, "--path-walk"); if (args->local) strvec_push(&cmd->args, "--local"); if (args->quiet) @@ -1212,6 +1215,8 @@ int cmd_repack(int argc, N_("pass --no-reuse-object to git-pack-objects")), OPT_INTEGER(0, "name-hash-version", &po_args.name_hash_version, N_("specify the name hash version to use for grouping similar objects by path")), + OPT_BOOL(0, "path-walk", &po_args.path_walk, + N_("pass --path-walk to git-pack-objects")), OPT_NEGBIT('n', NULL, &run_update_server_info, N_("do not run git-update-server-info"), 1), OPT__QUIET(&po_args.quiet, N_("be quiet")), diff --git a/t/perf/p5313-pack-objects.sh b/t/perf/p5313-pack-objects.sh index cd6dd3abb71..98748b0e203 100755 --- a/t/perf/p5313-pack-objects.sh +++ b/t/perf/p5313-pack-objects.sh @@ -55,23 +55,21 @@ test_all_with_args () { test_size "shallow pack size with $parameter" ' test_file_size out ' -} - -for version in 1 2 -do - export version - - test_all_with_args --name-hash-version=$version - test_perf "repack with --name-hash-version=$version" ' - git repack -adf --name-hash-version=$version + test_perf "repack with $parameter" ' + git repack -adf $parameter ' - test_size "repack size with --name-hash-version=$version" ' + test_size "repack size with $parameter" ' gitdir=$(git rev-parse --git-dir) && pack=$(ls $gitdir/objects/pack/pack-*.pack) && test_file_size "$pack" ' +} + +for version in 1 2 +do + test_all_with_args --name-hash-version=$version done test_all_with_args --path-walk -- gitgitgadget
