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Re: [PATCH 0/1] send-pack: set core.warnAmbiguousRefs=false

On 11/6/2018 2:44 PM, Jeff King wrote:
On Tue, Nov 06, 2018 at 11:13:47AM -0800, Derrick Stolee via GitGitGadget wrote:

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.
Some of this might be useful in the commit message. :)

The fix for this is simple: set core.warnAmbiguousRefs to false for this
specific call of git pack-objects coming from git send-pack. We don't want
to default it to false for all calls to git pack-objects, as it is valid to
pass ref names instead of object ids. This helps regain these seconds during
a push.
I don't think you actually care about the ambiguity check between refs
here; you just care about avoiding the ref check when we've seen (and
are mostly expecting) a 40-hex sha1. We have a more specific flag for
that: warn_on_object_refname_ambiguity.

And I think it would be OK to enable that all the time for pack-objects,
which is plumbing that does typically expect object names. See prior art
in 25fba78d36 (cat-file: disable object/refname ambiguity check for
batch mode, 2013-07-12) and 4c30d50402 (rev-list: disable object/refname
ambiguity check with --stdin, 2014-03-12).
Thanks for these pointers. Helps to know there is precedent for shutting down the
behavior without relying on "-c" flags.

Whenever I see a change like this to the pack-objects invocation for
send-pack, it makes me wonder if upload-pack would want the same thing.

It's a moot point if we just set the flag directly in inside
pack-objects, though.
I'll send a v2 that does just that.