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

[Jeff King <peff@peff.net>]

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).

> Derrick Stolee (1):
>   send-pack: set core.warnAmbiguousRefs=false
> 
>  send-pack.c | 2 ++
>  1 file changed, 2 insertions(+)

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.

-Peff