Slow git pushes: sitting 1 minute in pack-objects

[Stephen Morton <stephen.c.morton@gmail.com>]

(Apologies, after a day I'm cross-posting from git.users. I think the question
is maybe too technical for that group.)

I'm experiencing very slow git pushes. On the order of 1 minute to push a
trivial one-line change. When I set GIT_TRACE=1, I see that it seems to be
taking a lot of time in the pack-objects phase.

Others are not seeing this with the same repo, but I'm the only one working
in a VM.

```
~/ws/git/repo.1/repo > date; git push mortons; date
Wed Mar  4 15:03:11 EST 2015
15:03:11.086758 git.c:349               trace: built-in: git 'push' 'mortons'
15:03:11.126665 run-command.c:341       trace: run_command: 'ssh' '-p'
'7999' 'git@privacy.privacy' 'git-receive-pack
'\''~mortons/repo.git'\'''
15:03:20.383341 run-command.c:341       trace: run_command:
'pack-objects' '--all-progress-implied' '--revs' '--stdout' '--thin'
'--delta-base-offset' '--progress'
15:03:20.383945 exec_cmd.c:134          trace: exec: 'git'
'pack-objects' '--all-progress-implied' '--revs' '--stdout' '--thin'
'--delta-base-offset' '--progress'
15:03:20.385168 git.c:349               trace: built-in: git
'pack-objects' '--all-progress-implied' '--revs' '--stdout' '--thin'
'--delta-base-offset' '--progress'
Counting objects: 4, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 20.86 KiB | 0 bytes/s, done.
Total 4 (delta 0), reused 0 (delta 0)
To ssh://git@privacy.privacy:7999/~mortons/repo.git
   5fe662f..a137bda  my_branch -> my_branch
Wed Mar  4 15:04:22 EST 2015

```

After it was slow at first, I tried setting these which did not help

repack.writebitmaps=true
pack.windowmemory=100m



Details:
git version 2.1.4
OS: CentOS 6.6 64-bit in a VM.
repo size: huge. 6 GB .git directory, around 800 MB working tree.
VM has 8 MB RAM and 8 cores.
CPU: i7, 8 core (4 cores hyperthreaded)

It is an ext4 filesystem on the guest linux drive.
    On the host side, it is a .vmdk file and the virtualization software used is
virtualbox. While the drive is dynamically allocated, after I ran into
this issue,
    I used fallocate to create a 50GB dummy file and then delete it to
ensure that
there was headroom in the drive and that dynamic allocation slowness was not the
issue, and subsequent pushes were still slow.
    I have not experienced any filesystem slowness issues in the
months I've been using this vm.


Any ideas? I'm evaluating a move to git and this is the kind of thing
that could derail it.
Thanks,

Stephen