* Re: Performance regression in `git log --follow`
2026-10-13 13:55 Performance regression in `git log --follow` Patrick Steinhardt
@ 2026-10-13 21:24 ` SZEDER Gábor
0 siblings, 0 replies; 2+ messages in thread
From: SZEDER Gábor @ 2026-10-13 21:24 UTC (permalink / raw)
To: Patrick Steinhardt; +Cc: git, Miklos Vajna
On Tue, Oct 13, 2026 at 03:55:01PM +0200, Patrick Steinhardt wrote:
> Hi,
>
> a user on IRC reported a performance regression in linux.git:
>
> $ git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> $ git -C linux log --stat --follow -- include/linux/rsc_table.h
>
> I was able to reproduce the performance regression: while this command
> takes roughly 33 seconds with Git 2.55, it takes almost 8 minutes with
> Git 2.56.
>
> I've bisected the regression to 304812ed33 (log: improve --follow
> following renames for non-linear history, 2026-06-22). I don't have time
> right now to debug any further, so I decided to just forward the report
> to the mailing list. I've also Cc'd the original author of the above
> commit.
The performance regression is clearly noticable in the git repository,
though the slowdown is not as pronounced as in linux.git:
Benchmark 1: ./304812ed33^/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh
Time (mean ± σ): 3.275 s ± 0.022 s [User: 3.157 s, System: 0.116 s]
Range (min … max): 3.240 s … 3.313 s 10 runs
Benchmark 2: ./304812ed33/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh
Time (mean ± σ): 8.825 s ± 0.029 s [User: 8.554 s, System: 0.264 s]
Range (min … max): 8.769 s … 8.864 s 10 runs
Summary
'./304812ed33^/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh' ran
2.69 ± 0.02 times faster than './304812ed33/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh'
That's 5.5 seconds of runtime difference, and with a bit of trace
performance hack[1] it turns out that most of it is spent in a call to
propagate_follow_pathspec_to_parent() to compare the current commit
with its second...N-th parent:
$ GIT_TRACE_PERFORMANCE=1 ./304812ed33/bin/git --no-pager log --follow --oneline v2.56.0 t/valgrind/valgrind.sh >/dev/null
21:59:30.465592 read-cache.c:2364 performance: 0.000626524 s: read cache .git/index
21:59:39.246697 log-tree.c:1158 performance: 1.374980154 s: first parent propagate_follow_pathspec_to_parent()
21:59:39.247164 log-tree.c:1159 performance: 4.448692273 s: N-th parents propagate_follow_pathspec_to_parent()
21:59:39.247192 trace.c:416 performance: 8.799236172 s: git command: ./304812ed33/bin/git --no-pager log --follow --oneline v2.56.0 t/valgrind/valgrind.sh
Interestingly, although line-level log performs its own rename
tracking, it's still notably faster than "--follow" even before this
perf regression (that "-L1,40" means "track the whole file"):
Benchmark 1: ./304812ed33^/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh
Time (mean ± σ): 3.260 s ± 0.019 s [User: 3.140 s, System: 0.118 s]
Range (min … max): 3.219 s … 3.284 s 10 runs
Benchmark 2: ./304812ed33^/bin/git --no-pager log --oneline --no-patch -L1,40:t/valgrind/valgrind.sh v2.56.0
Time (mean ± σ): 1.454 s ± 0.014 s [User: 1.369 s, System: 0.084 s]
Range (min … max): 1.441 s … 1.481 s 10 runs
Summary
'./304812ed33^/bin/git --no-pager log --oneline --no-patch -L1,40:t/valgrind/valgrind.sh v2.56.0' ran
2.24 ± 0.02 times faster than './304812ed33^/bin/git --no-pager log --oneline --no-patch --follow v2.56.0 t/valgrind/valgrind.sh'
The reason for this might be:
- If line-level log finds that the interesting path is not modified
between a merge commit and its first parent, then it doesn't
perform a diff between that merge commit and its second..N-th
parent.
"--follow" seems to performs a diff between a merge commit and all
its parents.
- Line-level log checks for renames only when the path is not
present in the parent commit.
"--follow" seems to always look for renames.
[1] It really is just a hack:
diff --git a/log-tree.c b/log-tree.c
index 83a3c4bf9b..fd4824fd3e 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -34,6 +34,7 @@
#include "wildmatch.h"
#include "write-or-die.h"
#include "pager.h"
+#include "trace.h"
static struct decoration name_decoration = { "object names" };
static int decoration_loaded;
@@ -1144,6 +1145,8 @@ static void free_follow_pathspec_slot(char **slot)
FREE_AND_NULL(*slot);
}
+static uint64_t t_first = 0, t_nth = 0;
+
void release_follow_pathspec_slab(struct rev_info *opt)
{
if (!opt->follow_pathspec_slab)
@@ -1151,6 +1154,9 @@ void release_follow_pathspec_slab(struct rev_info *opt)
deep_clear_follow_pathspec_slab(opt->follow_pathspec_slab,
free_follow_pathspec_slot);
FREE_AND_NULL(opt->follow_pathspec_slab);
+
+ trace_performance(t_first, "first parent propagate_follow_pathspec_to_parent()");
+ trace_performance(t_nth, "N-th parents propagate_follow_pathspec_to_parent()");
}
/* Compute a path to follow in parent, if there is one */
@@ -1304,9 +1310,18 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
struct commit_list *parents = get_saved_parents(opt, commit);
if (parents && parents->next) {
struct commit_list *p;
- for (p = parents; p; p = p->next)
+ for (p = parents; p; p = p->next) {
+ if (p == parents)
+ t_first -= getnanotime();
+ else
+ t_nth -= getnanotime();
propagate_follow_pathspec_to_parent(opt, commit,
p->item);
+ if (p == parents)
+ t_first += getnanotime();
+ else
+ t_nth += getnanotime();
+ }
} else if (parents) {
remember_follow_pathspec(opt, parents->item,
pathspec_single_path(&opt->diffopt.pathspec));
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