I tried on the local disk as well and it didn't help. I managed to
find a SUSE11 machine and tried it there but no luck so I think we can
eliminate NFS and OS as significant factors now.

I ran with perf and here's the report:

ESC[31m    69.07%ESC[m              git  /lib64/libc-2.11.1.so
                                         [.] memcpy
ESC[31m    12.33%ESC[m              git
<prefix>/git-1.8.0.rc2.suse11/bin/git                           [.]
ESC[31m     5.11%ESC[m              git
<prefix>/zlib/local/lib/libz.so.1.2.5                           [.]
ESC[32m     2.61%ESC[m              git
<prefix>/zlib/local/lib/libz.so.1.2.5                           [.]
ESC[32m     1.98%ESC[m              git  /lib64/libc-2.11.1.so
                                         [.] _int_malloc
ESC[32m     0.86%ESC[m              git  [kernel]
                                         [k] clear_page_c

Does this help? Machine has 396GB of RAM if it matters.

On Thu, Nov 8, 2012 at 4:33 PM, Jeff King <p...@peff.net> wrote:
> 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.
> -Peff
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