[PATCH 0/1] builtin/pack-objects.c: avoid iterating all refs

This is a small patch for git-pack-objects which will help server side performance on repositories with lots of refs. I will post a related but slightly larger patch for ls-refs.c in a separate thread.

The back story is in https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/746 but I will try to summarize it here.

We have a particular Gitaly (Git RPC) server at GitLab that has a very homogenous workload, dominated by CI. While trying to reduce CPU utilization on the server we configured CI to fetch with the '--no-tags' option. This had an unexpectedly large impact so I started looking closer at why that may be.

What I learned is that by default, a fetch ends up using the '--include-tag' command line option of git-pack-objects. This causes git-pack-objects to iterate through all the tags of the repository to see if any should be included in the pack because they point to packed objects. The problem is that this "iterate through all the tags" uses for_each_ref which iterates through all references in the repository, and in doing so loads each associated object into memory to check if the ref is broken. But all we need for '--include-tag' is to iterate through refs/tags/.

On the repo we were testing this on, there are about 500,000 refs but only 2,000 tags. So we had to load a lot of objects just for the sake of '--include-tag'. It was common to see more than half the CPU time in git-pack-objects being spent in do_for_each_ref, and that in turn was dominated by ref_resolves_to_object.

So, I think it would be nice to just iterate over those 2,000 tags and not load 500,000 objects outside refs/tags we already know we don't care about.

Jacob Vosmaer (1):
  builtin/pack-objects.c: avoid iterating all refs

 builtin/pack-objects.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)