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
