Seeing high CPU usage during git-upload-pack

[David Ebbo <david.ebbo@gmail.com>]

We’re using git-upload-pack in a simple git server. When the repo is
on a slow share, we’re noticing that git-upload-pack is hugging the
CPU during the whole pack-objects operation.

More details:

- git-upload-pack gets launched (with params git-upload-pack
--stateless-rpc //path/to/repo)
- it itself launches a git process (with params pack-objects --revs
--all --stdout --progress --delta-base-offset). This git process is
the one that does all the disk I/O, but it's not the one using up the
CPU time.
- git-upload-pack appears to be waiting for the git process, and is at
high CPU during that time, suggesting that it's doing some kind of
busy wait. To further test that, I stopped the git process in the
debugger, preventing it from making progress. At that point,
git-upload-pack pegs the CPU forever.

Would someone familiar with the sources be able to comment on this
busy wait behavior, and on whether it could conceivably be done in a
way that doesn’t use up some much CPU?

Thanks,
David