Git push performance problems with ~100K refs

Hello,

I am back to talk about git performance with lots of refs: ~100K. This time I am investigating pushes to a repo with about ~100K ref, a gerrit mirror which includes the gerrit changes. When I push just a simple one line change to a file, the push takes about ~43s. If I delete the changes from the destination repo, this push takes about 6s. This seems rather excessive to me, in fact given that the repo with 100K refs has more data in it and is more likely to have the objects I am pushing in it, if things are done right, it should be a faster push (in theory).

So, upon early investigation, I noticed that the time to push seems mostly determined by the receiving end which is processing all out for 100% on a CPU. During this time period, the receiving end git commands look like this:

  git-receive-pack path/to/repo.git

and:

  git rev-list --objects --stdin --not --all

The latter of these two commands is the one burning CPU.

Does anyone have any hints as to what might be wrong with the receiving end algorithm that would cause a small change to use so much CPU? Is there anything that can be done about it? I noticed that the --all option will effectively feed all the 100K refs to rev-list, is this really necessary? Are there any tests that I can perform to help debug this?

I am using git 1.7.8.3 and I also tried 1.7.10.rc3, same results.

Thanks,

-Martin