Subject: Re: [PATCH] bitmaps: don't recurse into trees already in the bitmap

From: Patrick Steinhardt <[email-masked]>
Date: Fri, 18 Jun 2021 15:35:05 +0200

On Fri, Jun 18, 2021 at 08:59:59AM -0400, Jeff King wrote:
> On Wed, Jun 16, 2021 at 02:31:04PM +0200, Patrick Steinhardt wrote:
> 
> > > These many-refs scenarios make sense as something that is difficult to
> > > verify using a single fork of an open-source project, but is common in
> > > many closed-source projects that do not use forking to reduce the ref
> > > count per repo.
> > 
> > Agreed. What I typically do to emulate this is to use some version of
> > following command to create refs for "$n" commits.
> > 
> >     git log --all --format="tformat:create refs/commit/%h %H" |
> >         shuf | head -n "$n" | git update-ref --stdin
> > 
> > It's obviously not ideal given that resulting refs are distributed at
> > random. But combined with a sufficiently large repo, it's still helped
> > me at times to reproduce adverse performance at times.
> 
> Yeah, I've done something similar. But I'd caution people into reading
> too much into performance tests from something like that. What I've
> found over the years is that traversal and bitmap performance can be
> somewhat sensitive to tree shape and bitmap placement (which in turn is
> often influenced by ref placement, if you are using delta islands or the
> recently added pack.preferBitmapTips).
> 
> More than once I've spent many head-scratching hours trying to determine
> why some real-world repo performs better or worse than expected. :)

I couldn't agree more. I've also had my fair share of weird performance
characteristics in completely unexpected ways. Unfortunately, it has
made me become quite cautious about bitmaps given that they've already
caused their fair share of perfomance regressions.

But your work here actually encourages me to give it another try soonish
and see what kind of repo shapes make them explode this time.

Patrick