On Thu, Aug 10, 2023 at 06:08:34AM +0000, Ronan Pigott wrote: > I am interested in git performance today and can't figure out what's going on > here. I was wondering why my git-fetch might be slow in an up-to-date repo: > > $ git pull > Already up to date. > $ time git fetch origin master > From https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux > * branch master -> FETCH_HEAD > git fetch origin master 0.13s user 0.06s system 10% cpu 1.705 total > > GIT_TRACE_CURL shows it spends most of the time transfering (all) tags from the > remote. It's much faster with --no-tags: > > $ time git fetch -n origin master > From https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux > * branch master -> FETCH_HEAD > git fetch -n origin master 0.11s user 0.03s system 36% cpu 0.383 total > > But I don't have tagOpt set: > > $ git config remote.origin.tagOpt || echo $? > 1 > > And the remote doesn't have to send me any commits, so I don't see why I should > receive any tags at all. Why might I be receiving so many tags? You didn't define "receiving tags", but I assume you just mean that you saw the tag names and object ids in the trace output. From the output above, it looks like no actual tag objects were transferred. And the answer, then, is that this is how the Git protocol works. The server says "here are all the refs I know about", then the client decides what it wants from that list and asks the server to send the necessary objects, after which it updates its local refs. So the server will necessarily send all of the tags. Only the client knows what it already has and whether any of them are new. And in the default mode, which will fetch tags that point to commits we have, it is checking each such new tag to see if it is worth fetching. Even if we did not fetch new commits, we might see new tags that point to existing commits. When you use "--no-tags", that explicitly says "do not bother with tags at all". Recent versions of Git have a protocol extension where the client can say "I am only interested in refs/heads/master; don't bother telling me about other stuff". Since the client knows we do not care about tags, it can use that extension to get a much smaller ref advertisement from the server. -Peff
