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packfile: fix perf regression with many packs 

Since 589127caa7 (packfile: move list of packs into the packfile
store, 2025-10-30), there is a performance regression when many
packfiles need to be loaded: `packfile_store_add_pack()` now calls
`packfile_list_remove_internal()` to detect whether the packfile was
_already_ in the list, and if so, move it to the end of the list. This
function linearly scans the existing list before every insertion. Newly
loading N packs therefore has complexity O(N²).

In one reported use case (https://github.com/microsoft/git/issues/970),
N equals 37,815 and caused a slow-down of a simple `git rev-parse
--short HEAD` (which is regularly executed as part of `GIT_PS1`) from
0.4s to 4.5s.

Let's fix this by establishing a fast path for known-new packfiles.

The keen reader will note that there is currently only a single,
"known-new" caller of the `packfile_list_append()` function, and wonder
why not simply remove this check whether the packfile already exists in
the list? Originally, when above-mentioned commit introduced that logic,
there was a second caller in `prepare_midx()`, which would have required
that check, but that caller was removed in 6aff1f25a0 (packfile:
always add packfiles to MRU when adding a pack, 2025-10-30). Still, the
function is declared in a header file, and to avoid any problems with
in-flight or downstream callers, it is safer to extend the signature to
be explicit whether or not to skip that check.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
main
Johannes Schindelin 2026-08-13 14:56:49 +00:00 committed by Junio C Hamano
parent f85a7e6620
commit e4621a0169
4 changed files with 10 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
packfile-list.c
View File

@ -57,11 +57,12 @@ void packfile_list_prepend(struct packfile_list *list, struct packed_git *pack)
list->tail = entry;
}

void packfile_list_append(struct packfile_list *list, struct packed_git *pack)
void packfile_list_append(struct packfile_list *list, struct packed_git *pack,
int skip_dup_check)
{
struct packfile_list_entry *entry;

entry = packfile_list_remove_internal(list, pack);
entry = skip_dup_check ? NULL : packfile_list_remove_internal(list, pack);
if (!entry) {
entry = xmalloc(sizeof(*entry));
entry->pack = pack;

3
packfile-list.h
View File

@ -15,7 +15,8 @@ struct packfile_list_entry {
void packfile_list_clear(struct packfile_list *list);
void packfile_list_remove(struct packfile_list *list, struct packed_git *pack);
void packfile_list_prepend(struct packfile_list *list, struct packed_git *pack);
void packfile_list_append(struct packfile_list *list, struct packed_git *pack);
void packfile_list_append(struct packfile_list *list, struct packed_git *pack,
int skip_dup_check);

/*
* Find the pack within the "packs" list whose index contains the object

2
packfile.c
View File

@ -781,7 +781,7 @@ void packfile_store_add_pack(struct odb_source_packed *store,
if (pack->pack_fd != -1)
pack_open_fds++;

packfile_list_append(&store->packs, pack);
packfile_list_append(&store->packs, pack, 1);
strmap_put(&store->packs_by_path, pack->pack_name, pack);
}


4
t/perf/p5303-many-packs.sh
View File

@ -141,4 +141,8 @@ test_perf "load 10,000 packs" '
git rev-parse --verify "HEAD^{commit}"
'

test_perf "abbreviate with 10,000 packs" '
git rev-parse --short HEAD
'

test_done