[PATCH 2/2] builtin/repack: don't regenerate MIDX unless needed
- From
Patrick Steinhardt <ps@pks.im>
- Date
- Dec 8, 2025, 18:27 UTC
- Message-ID
- <20251208-pks-skip-noop-rewrite-v1-2-430d52dba9f0@pks.im>
- In-Reply-To
- <20251208-pks-skip-noop-rewrite-v1-0-430d52dba9f0@pks.im>
When calling `git repack --write-midx` we will unconditionally rewrite the multi-pack index. This is of course expected in the case where the layout of packfiles has changed. But when the layout of packfiles did not change it's of course a potentially-large waste of time.
With our default maintenance strategy this isn't really that much of a problem, as git-repack(1) would only be called by git-gc(1) in case we know that the layout _will_ change. But that is changing with geometric repacking, where it is the responsibility of git-repack(1) itself to determine whether or not any packs need to be merged together. So while git-repack happily declares that there is "Nothing new to pack.", we end up rewriting the MIDX.
This issue can be demonstrated trivially with a benchmark in the Git repository: executing `git repack --geometric=2 --write-midx -d` in the Git repository takes more than 3 seconds only to end up with the same multi-pack index as we already had before.
Address this issue by introducing a new function that determines whether a rewrite of the MIDX would cause any user-visible changes. This covers the following cases:
- No multi-pack index exists at all.
- The user asked us to write a bitmap, and we don't have any.
- The request preferred pack is different than the one that we have.
- The packfiles covered by the MIDX are changing.
Only if any of these conditions trigger we decide to write a new MIDX. This allows us to significantly reduce the time for repacks that end up doing nothing:
Benchmark 1: git repack --geometric=2 --write-midx -d
Time (mean ± σ): 3.183 s ± 0.078 s [User: 2.924 s, System: 0.219 s]
Range (min … max): 2.985 s … 3.260 s 10 runs Benchmark 2: ./git repack --geometric=2 --write-midx -d
Time (mean ± σ): 102.5 ms ± 1.0 ms [User: 89.3 ms, System: 12.7 ms]
Range (min … max): 101.3 ms … 105.3 ms 28 runs Summary
./git repack --geometric=2 --write-midx -d ran
31.06 ± 0.82 times faster than git repack --geometric=2 --write-midx -dSigned-off-by: Patrick Steinhardt <ps@pks.im> --- repack-midx.c | 90 +++++++++++++++++++++++++++++++++++++++++++++ t/t5319-multi-pack-index.sh | 42 +++++++++++++++++++++ t/t7703-repack-geometric.sh | 80 ++++++++++++++++++++++++++++++++++++++++ 3 files changed, 212 insertions(+)
diff --git a/repack-midx.c b/repack-midx.c index 74bdfa3a6e..4e47f7c713 100644 --- a/repack-midx.c +++ b/repack-midx.c @@ -2,6 +2,7 @@ #include "repack.h" #include "hash.h" #include "hex.h" +#include "midx.h" #include "odb.h" #include "oidset.h" #include "pack-bitmap.h" @@ -283,6 +284,92 @@ static void remove_redundant_bitmaps(struct string_list *include, strbuf_release(&path); } +static bool midx_needs_update(struct repack_write_midx_opts *opts, + struct string_list *include, + struct packed_git *new_preferred) +{ + struct multi_pack_index *midx; + uint32_t preferred_pack_id; + bool needed = true; + + /* + * If there is no multi-pack index we obviously need to generate a new + * one. Note that we cannot use `get_multi_pack_index()` here, as we + * might be operating with a closed object database. + */ + midx = load_multi_pack_index(opts->existing->repo->objects->sources); + if (!midx) + goto out; + + /* + * If we're instructed to write a bitmap we need to verify whether we + * already got one. + */ + if (opts->write_bitmaps) { + struct bitmap_index *bitmap = prepare_midx_bitmap_git(midx); + bool bitmap_exists = bitmap && bitmap_is_midx(bitmap); + free_bitmap_index(bitmap); + if (!bitmap_exists) + goto out; + } + + /* + * If we're asked to generate the MIDX with a preferred pack we need to + * verify that the current prepared pack matches the desired one. We + * can only determine this in the case where we write bitmaps though, + * so we ignore this setting otherwise. + */ + if (new_preferred && opts->write_bitmaps) { + struct packed_git *old_preferred = NULL; + + if (!midx_preferred_pack(midx, &preferred_pack_id)) + old_preferred = nth_midxed_pack(midx, preferred_pack_id); + + if (!old_preferred) + goto out; + + if (strcmp(pack_basename(new_preferred), + pack_basename(old_preferred))) + goto out; + } + + /* + * Otherwise, we need to verify that the packs that are about to be + * included in the MIDX matches the currently covered packs. + */ + if (include->nr != opts->existing->midx_packs.nr) + goto out; + + string_list_sort(include); + string_list_sort(&opts->existing->midx_packs); + for (size_t i = 0; i < include->nr; i++) { + const char *include_name = include->items[i].string; + const char *existing_name = opts->existing->midx_packs.items[i].string; + size_t include_len = strlen(include_name); + size_t existing_len = strlen(existing_name); + + /* + * We track pack indices for the include list, but the + * packfiles themselves in the existing list. We thus need to + * strip these suffixes. + */ + if (ends_with(include_name, ".idx")) + include_len -= strlen(".idx"); + if (ends_with(existing_name, ".pack")) + existing_len -= strlen(".pack"); + if (include_len != existing_len) + goto out; + if (memcmp(include_name, existing_name, include_len)) + goto out; + } + + needed = false; + +out: + close_midx(midx); + return needed; +} + int write_midx_included_packs(struct repack_write_midx_opts *opts) { struct child_process cmd = CHILD_PROCESS_INIT; @@ -296,6 +383,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts) if (!include.nr) goto done; + if (!midx_needs_update(opts, &include, preferred)) + goto done; + cmd.in = -1; cmd.git_cmd = 1; diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index 9492a9737b..4adf67385a 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -366,6 +366,48 @@ test_expect_success 'preferred pack cannot be determined without bitmap' ' ) ' +test_midx_is_retained () { + test-tool chmtime =0 .git/objects/pack/multi-pack-index && + ls -l .git/objects/pack/multi-pack-index >expect && + git repack "$@" && + ls -l .git/objects/pack/multi-pack-index >actual && + test_cmp expect actual +} + +test_midx_is_rewritten () { + test-tool chmtime =0 .git/objects/pack/multi-pack-index && + ls -l .git/objects/pack/multi-pack-index >expect && + git repack --write-midx "$@" && + ls -l .git/objects/pack/multi-pack-index >actual && + ! test_cmp expect actual +} + +test_expect_success 'up-to-date multi-pack-index is retained' ' + test_when_finished "rm -fr midx-up-to-date" && + git init midx-up-to-date && + ( + cd midx-up-to-date && + + # Write the initial pack that contains the most objects. This + # will be the preferred pack. + test_commit first && + test_commit second && + git repack -Ad --write-midx && + test_midx_is_retained -Ad && + + # Writing a new bitmap index should cause us to regenerate the MIDX. + test_midx_is_rewritten -Ad --write-bitmap-index && + test_midx_is_retained -Ad --write-bitmap-index && + + # Ensure that writing a new packfile causes us to rewrite the index. + test_commit incremental && + test_midx_is_rewritten -d && + test_midx_is_retained -d + ) +' + +test_done + test_expect_success 'verify multi-pack-index success' ' git multi-pack-index verify --object-dir=$objdir ' diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh index 9fc1626fbf..980599961c 100755 --- a/t/t7703-repack-geometric.sh +++ b/t/t7703-repack-geometric.sh @@ -287,6 +287,86 @@ test_expect_success '--geometric with pack.packSizeLimit' ' ) ' +test_expect_success '--geometric --write-midx retains up-to-date MIDX without bitmap index' ' + test_when_finished "rm -fr repo" && + git init repo && + ( + cd repo && + test_commit initial && + + test_path_is_missing .git/objects/pack/multi-pack-index && + git repack --geometric=2 --write-midx --no-write-bitmap-index && + test_path_is_file .git/objects/pack/multi-pack-index && + test-tool chmtime =0 .git/objects/pack/multi-pack-index && + + ls -l .git/objects/pack/ >expect && + git repack --geometric=2 --write-midx --no-write-bitmap-index && + ls -l .git/objects/pack/ >actual && + test_cmp expect actual + ) +' + +test_expect_success '--geometric --write-midx regenerates MIDX when preferred pack changes' ' + test_when_finished "rm -fr repo" && + git init repo && + ( + cd repo && + + test_commit first && + test_commit second && + test_commit third && + git repack --geometric=2 --write-midx --write-bitmap-index && + test_commit fourth && + git repack --geometric=2 --write-midx --write-bitmap-index && + + ls .git/objects/pack/*.pack >packs && + test_line_count = 2 packs && + preferred_pack=$(test-tool read-midx --preferred-pack .git/objects) && + other_pack=$(ls .git/objects/pack/*.idx | grep -v "$preferred_pack") && + echo "$preferred_pack" && + echo "$other_pack" && + + # Rewrite the multi-pack index with the current preferred pack. + # git-repack(1) should decide to _not_ repack the MIDX in that + # case. This is mostly a sanity check to verify that the reason + # for the repack really only is the changed preferred pack. + rm -f .git/objects/pack/multi-pack-index* && + git multi-pack-index write --bitmap --preferred-pack="$preferred_pack" && + test-tool chmtime =0 .git/objects/pack/multi-pack-index && + ls -l .git/objects/pack/ >expect && + git repack --geometric=2 --write-midx --write-bitmap-index && + ls -l .git/objects/pack/ >actual && + test_cmp expect actual && + + # Rewrite the multi-pack index with a different preferred pack. + # This time around, git-repack(1) should decide to repack the + # MIDX to rectify the preferred pack. + rm -f .git/objects/pack/multi-pack-index* && + git multi-pack-index write --bitmap --preferred-pack="$(basename "$other_pack")" && + test-tool chmtime =0 .git/objects/pack/multi-pack-index && + ls -l .git/objects/pack/ >expect && + git repack --geometric=2 --write-midx --write-bitmap-index && + ls -l .git/objects/pack/ >actual && + ! test_cmp expect actual + ) +' + +test_expect_success '--geometric --write-midx retains up-to-date MIDX with bitmap index' ' + test_when_finished "rm -fr repo" && + git init repo && + test_commit -C repo initial && + + test_path_is_missing repo/.git/objects/pack/multi-pack-index && + git -C repo repack --geometric=2 --write-midx --write-bitmap-index && + test_path_is_file repo/.git/objects/pack/multi-pack-index && + test-tool chmtime =0 repo/.git/objects/pack/multi-pack-index && + + ls -l repo/.git/objects/pack/ >expect && + git -C repo repack --geometric=2 --write-midx --write-bitmap-index && + ls -l repo/.git/objects/pack/ >actual && + test_cmp expect actual +' + test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' ' test_when_finished "rm -fr shared member" &&
-- 2.52.0.270.g3f4935d65f.dirty