Re: [PATCH 1/1] delete multiple tags in a single transaction

From: Phil Hord <phil.hord@gmail.com>
Date: Aug 8, 2019, 23:43 UTC
Message-ID: <CABURp0p5xbsq+8UsFerMAY8EG-ndXgd19EUsHOgQG-dnDnTAgg@mail.gmail.com>
In-Reply-To: <CABPp-BFH++aJinkzg+qsZDRN6R5-E8LPCG_u+udZLW6o0MGBug@mail.gmail.com>

On Thu, Aug 8, 2019 at 11:15 AM Elijah Newren <newren@gmail.com> wrote:

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>
> On Wed, Aug 7, 2019 at 9:11 PM Phil Hord <phil.hord@gmail.com> wrote:
> >
> > From: Phil Hord <phil.hord@gmail.com>
> >
> > 'git tag -d' accepts one or more tag refs to delete, but each deletion
> > is done by calling `delete_ref` on each argv. This is painfully slow
> > when removing from packed refs. Use delete_refs instead so all the
> > removals can be done inside a single transaction with a single write.
>
> Nice, thanks for working on this.
>
> > I have a repo with 24,000 tags, most of which are not useful to any
> > developers. Having this many refs slows down many operations that
> > would otherwise be very fast. Removing these tags when they've been
> > accidentally fetched again takes about 30 minutes using delete_ref.
>
> I also get really slow times on a repo with ~20,000 tags (though order
> ~3 minutes rather than ~30, probably due to having an SSD on this
> machine) -- but ONLY IF the refs are packed first (git pack-refs
> --all).  If the refs are loose, it's relatively quick to delete a
> dozen thousand or so tags (order of a few seconds).  It might be worth
> mentioning in the commit message that this only makes a significant
> difference in the case where the refs are packed.

I'm also using an SSD but I still see about 10 tags per second being deleted with the current code (and packed-refs). I see that I'm CPU-bound, so I guess most of the time is spent searching through .git/packed-refs. Probably it will run faster as it progresses. I guess the 18,000 branches in my repo keep me on the wrong end of O(N).

My VM is on an all-flash storage array, but I can't say much about its write throughput since it's one VM among many.

Previously I thought I saw a significant speedup between v2.7.4 (on my development vm) and v2.22.0 (on my laptop). But this week I saw it was slow again on my laptop. I looked for the regression but didn't find anyone touching that code. Then I wrote this patch.

But it should have occurred to me while I was in the code that there is a different path for unpacked refs which could explain my previous speeds. I didn't think I had any unpacked refs, though, since every time I look in .git/refs for what I want, I find it relatively empty. I see 'git pack-refs --help' says that new refs should show up loose, but I can't say that has happened for me. Maybe a new clone uses packed-refs for *everything* and only newly fetched things are loose. Is that it? I guess since I seldom fetch tags after the first clone, it makes sense they would all be packed.

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> >     git tag -l feature/* | xargs git tag -d
> >
> > Removing the same tags using delete_refs takes less than 5 seconds.
>
> It appears this same bug also affects `git branch -d` when deleting
> lots of branches (or remote tracking branches) and they are all
> packed; could you apply the same fix there?

Will do.

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> In constrast, it appears that `git update-ref --stdin` is fast
> regardless of whether the refs are packed, e.g.
>    git tag -l feature/* | sed -e 's%^%delete refs/tags/%' | git
> update-ref --stdin
> finishes quickly (order of a few seconds).

Nice! That trick is going in my wiki for devs to use on their VMs. Thanks for that.