On Thu, Nov 08, 2012 at 04:16:59PM -0600, Uri Moszkowicz wrote:

> I ran "git cat-file commit some-tag" for every tag. They seem to be
> roughly uniformly distributed between 0s and 2s and about 2/3 of the
> time seems to be system. My disk is mounted over NFS so I tried on the
> local disk and it didn't make a difference.
> I have only one 1.97GB pack. I ran "git gc --aggressive" before.

Ah. NFS. That is almost certainly the source of the problem. Git will
aggressively mmap. I would not be surprised to find that RHEL4's NFS
implementation is not particularly fast at mmap-ing 2G files, and is
spending a bunch of time in the kernel servicing the requests.

Aside from upgrading your OS or getting off of NFS, I don't have a lot
of advice.  The performance characteristics you are seeing are so
grossly off of what is normal that using git is probably going to be
painful. Your 2s cat-files should be more like .002s. I don't think
there's anything for git to fix here.

You could try building with NO_MMAP, which will emulate it with pread.
That might fare better under your NFS implementation. Or it might be
just as bad.

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