send-pack: set core.warnAmbiguousRefs=false

I've been looking into the performance of git push for very large repos. Our users are reporting that 60-80% of git push time is spent during the "Enumerating objects" phase of git pack-objects.

A git push process runs several processes during its run, but one includes git send-pack which calls git pack-objects and passes the known have/wants into stdin using object ids. However, the default setting for core.warnAmbiguousRefs requires git pack-objects to check for ref names matching the ref_rev_parse_rules array in refs.c. This means that every object is triggering at least six "file exists?" queries.

When there are a lot of refs, this can add up significantly! My PerfView trace for a simple push measured 3 seconds spent checking these paths.

The fix is to set the global warn_on_object_refname_ambiguity to 0 for the section that is performing these object reads.

In addition to this patch submission, we are looking into merging it into our fork sooner [1].

[1] https://github.com/Microsoft/git/pull/67

Changes in V2: Instead of using the "-c" flag from send-pack, just set the global. I left the name of the cover letter the same to not confuse anyone viewing the message without threading.

Derrick Stolee (1):
pack-objects: ignore ambiguous object warnings

 builtin/pack-objects.c | 6 ++++++
 1 file changed, 6 insertions(+)

base-commit: cae598d9980661a978e2df4fb338518f7bf09572
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-68%2Fderrickstolee%2Fsend-pack-config-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-68/derrickstolee/send-pack-config-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/68

Range-diff vs v1:

 1:  1ef2c51550 < -:  ---------- send-pack: set core.warnAmbiguousRefs=false
 -:  ---------- > 1:  002868ee6b pack-objects: ignore ambiguous object warnings

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