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commit: convert pop_most_recent_commit() to prio_queue

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pop_most_recent_commit() calls commit_list_insert_by_date() for parent
commits, which is itself called in a loop.  This can lead to quadratic
complexity if there are many merges.  Replace the commit_list with a
prio_queue to ensure logarithmic worst case complexity and convert all
three users.

Add a performance test that exercises one of them using a pathological
history that consists of 50% merges and 50% root commits to demonstrate
the speedup:

Test                          v2.50.1           HEAD
----------------------------------------------------------------------
1501.2: rev-parse ':/65535'   2.48(2.47+0.00)   0.20(0.19+0.00) -91.9%

Alas, sane histories don't benefit from the conversion much, and
traversing Git's own history takes a 1% performance hit on my machine:

   $ hyperfine -w3 -L git ./git_2.50.1,./git '{git} rev-parse :/^Initial.revision'
   Benchmark 1: ./git_2.50.1 rev-parse :/^Initial.revision
     Time (mean ± σ):      1.071 s ±  0.004 s    [User: 1.052 s, System: 0.017 s]
     Range (min … max):    1.067 s …  1.078 s    10 runs

   Benchmark 2: ./git rev-parse :/^Initial.revision
     Time (mean ± σ):      1.079 s ±  0.003 s    [User: 1.060 s, System: 0.017 s]
     Range (min … max):    1.074 s …  1.083 s    10 runs

   Summary
     ./git_2.50.1 rev-parse :/^Initial.revision ran
       1.01 ± 0.00 times faster than ./git rev-parse :/^Initial.revision

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
René Scharfe
2025-07-18 11:39:06 +02:00
committed by Junio C Hamano
parent 16bd9f20a4
commit d6ec08788e
7 changed files with 100 additions and 21 deletions
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7
commit.c
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@@ -31,6 +31,7 @@
#include "parse.h"
#include "object-file.h"
#include "object-file-convert.h"
#include "prio-queue.h"
static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
@@ -738,17 +739,17 @@ void commit_list_sort_by_date(struct commit_list **list)
commit_list_sort(list, commit_list_compare_by_date);
}
struct commit *pop_most_recent_commit(struct commit_list **list,
struct commit *pop_most_recent_commit(struct prio_queue *queue,
unsigned int mark)
{
struct commit *ret = pop_commit(list);
struct commit *ret = prio_queue_get(queue);
struct commit_list *parents = ret->parents;
while (parents) {
struct commit *commit = parents->item;
if (!repo_parse_commit(the_repository, commit) && !(commit->object.flags & mark)) {
commit->object.flags |= mark;
commit_list_insert_by_date(commit, list);
prio_queue_put(queue, commit);
}
parents = parents->next;
}