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revision: avoid hitting packfiles when commits are in commit-graph

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When queueing references in git-rev-list(1), we try to optimize parsing
of commits via the commit-graph. To do so, we first look up the object's
type, and if it is a commit we call `repo_parse_commit()` instead of
`parse_object()`. This is quite inefficient though given that we're
always uncompressing the object header in order to determine the type.
Instead, we can opportunistically search the commit-graph for the object
ID: in case it's found, we know it's a commit and can directly fill in
the commit object without having to uncompress the object header.

Expose a new function `lookup_commit_in_graph()`, which tries to find a
commit in the commit-graph by ID, and convert `get_reference()` to use
this function. This provides a big performance win in cases where we
load references in a repository with lots of references pointing to
commits. The following has been executed in a real-world repository with
about 2.2 million refs:

    Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.458 s ±  0.044 s    [User: 4.115 s, System: 0.342 s]
      Range (min … max):    4.409 s …  4.534 s    10 runs

    Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      3.089 s ±  0.015 s    [User: 2.768 s, System: 0.321 s]
      Range (min … max):    3.061 s …  3.105 s    10 runs

    Summary
      'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran
        1.44 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Patrick Steinhardt
2021-08-09 10:12:03 +02:00
committed by Junio C Hamano
parent 809ea28f80
commit f559d6d45e
3 changed files with 40 additions and 10 deletions
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18
revision.c
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@ -360,20 +360,18 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
unsigned int flags)
{
struct object *object;
struct commit *commit;
/*
* If the repository has commit graphs, repo_parse_commit() avoids
* reading the object buffer, so use it whenever possible.
* If the repository has commit graphs, we try to opportunistically
* look up the object ID in those graphs. Like this, we can avoid
* parsing commit data from disk.
*/
if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
struct commit *c = lookup_commit(revs->repo, oid);
if (!repo_parse_commit(revs->repo, c))
object = (struct object *) c;
else
object = NULL;
} else {
commit = lookup_commit_in_graph(revs->repo, oid);
if (commit)
object = &commit->object;
else
object = parse_object(revs->repo, oid);
}
if (!object) {
if (revs->ignore_missing)