Fix performance regression in v2.55 related to having many packfiles - #971
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Pull request overview
This PR addresses a performance regression when repositories contain very large numbers of packfiles by removing an O(n) “dedupe/removal” step from the hot path used while initially loading packfiles into a packfile_store. It also extends the perf suite to cover the rev-parse --short scenario that motivated the report.
Changes:
- Add a fast-path helper to append a new pack entry to a
packfile_listwithout scanning/removing. - Use that helper when adding packs to a
packfile_store, avoiding quadratic behavior during bulk pack discovery/loading. - Add a perf test that measures
git rev-parse --short HEADwith 10,000 packs.
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| File | Description |
|---|---|
t/perf/p5303-many-packs.sh |
Adds a perf benchmark for rev-parse --short under extreme packfile counts. |
packfile.c |
Introduces packfile_list_append_new() and uses it in packfile_store_add_pack() to avoid O(n²) pack insertion behavior. |
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Matthew John Cheetham (mjcheetham)
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Aug 5, 2026
Since 589127c (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, if 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 (#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. In another, heavily exercised CI scenario, clone times increased from under 2 minutes to over half an hour. 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 6aff1f2 (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>
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When there are many packfiles in the repository (or in an alternate of it), a regression introduced into Git v2.53 introduced complexity quadratic in the number of the packfiles. In one instance, causing a slow-down of a simple
git rev-parse --short HEAD(as used inGIT_PS1) from 0.4s to 4.5s. Let's fix that by avoiding the quadratic behavior in the most common case: loading the packfiles afresh.In another (internally-reported) instance, clone times that increased to over 30 minutes were reported to reduce to under 2 minutes with this patch.
This closes #970.