On Mon, May 16, 2022 at 7:01 PM Junio C Hamano <gitster@xxxxxxxxx> wrote: > > "Orgad Shaneh via GitGitGadget" <gitgitgadget@xxxxxxxxx> writes: > > > From: Orgad Shaneh <orgads@xxxxxxxxx> > > > > This check was introduced in 8ee5d73137f (Fix fetch/pull when run without > > --update-head-ok, 2008-10-13) in order to protect against replacing the ref > > of the active branch by mistake, for example by running git fetch origin > > master:master. > > > > It was later extended in 8bc1f39f411 (fetch: protect branches checked out > > in all worktrees, 2021-12-01) to scan all worktrees. > > > > This operation is very expensive (takes about 30s in my repository) when > > there are many tags or branches, and it is executed on every fetch, even if > > no local heads are updated at all. > > > > Limit it to protect only refs/heads/* to improve fetch performance. > > The point of the check is to prevent the index+working tree in the > worktrees to go out of sync with HEAD, and HEAD by definition can > point only into refs/heads/*, this change should be OK. > > It is surprising find_shared_symref() is so expensive, though. If > you have a dozen worktrees linked to the current repository, there > are at most a dozen HEAD that point at various refs in refs/heads/ > namespace. Even if you need to check a thousand ref_map elements, > it should cost almost nothing if you build a hashmap to find matches > with these dozen HEADs upfront, no? I also had this idea, but I'm not familiar enough with the codebase to implement it. I see you already started that. > Another thing that is surprising is that you say this loop is > expensive when there are many tags or branches. Do you mean it is > expensive when there are many tags and branches that are updated, or > it is expensive to merely have thousands of dormant tags and > branches? If the latter, I wonder if it is sensible to limit the > check only to the refs that are going to be updated. It's expensive even when *nothing* is updated. I have a repo with 44K tags, 13K of the tags are annotated, 134 remote branches and 4 worktrees (except the main repo) with 33 local branches. I counted the calls to find_shared_symref - it was called 35755 times, and refs_read_raw_ref was called 357585 times. - Orgad
