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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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13
fetch-pack.c
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@@ -34,6 +34,7 @@
#include "commit-graph.h"
#include "sigchain.h"
#include "mergesort.h"
#include "prio-queue.h"
static int transfer_unpack_limit = -1;
static int fetch_unpack_limit = -1;
@@ -600,7 +601,7 @@ done:
return count ? retval : 0;
}
static struct commit_list *complete;
static struct prio_queue complete = { compare_commits_by_commit_date };
static int mark_complete(const struct object_id *oid)
{
@@ -608,7 +609,7 @@ static int mark_complete(const struct object_id *oid)
if (commit && !(commit->object.flags & COMPLETE)) {
commit->object.flags |= COMPLETE;
commit_list_insert(commit, &complete);
prio_queue_put(&complete, commit);
}
return 0;
}
@@ -625,9 +626,12 @@ static int mark_complete_oid(const char *refname UNUSED,
static void mark_recent_complete_commits(struct fetch_pack_args *args,
timestamp_t cutoff)
{
while (complete && cutoff <= complete->item->date) {
while (complete.nr) {
struct commit *item = prio_queue_peek(&complete);
if (item->date < cutoff)
break;
print_verbose(args, _("Marking %s as complete"),
oid_to_hex(&complete->item->object.oid));
oid_to_hex(&item->object.oid));
pop_most_recent_commit(&complete, COMPLETE);
}
}
@@ -797,7 +801,6 @@ static void mark_complete_and_common_ref(struct fetch_negotiator *negotiator,
refs_for_each_rawref(get_main_ref_store(the_repository),
mark_complete_oid, NULL);
for_each_cached_alternate(NULL, mark_alternate_complete);
commit_list_sort_by_date(&complete);
if (cutoff)
mark_recent_complete_commits(args, cutoff);
}