Re: [PATCH] packfile: fix perf regression with many packs

Hi Junio,

On Wed, 12 Aug 2026, Junio C Hamano wrote:

> "Johannes Schindelin via GitGitGadget" <[email protected]>
> writes:
> 
> > In one reported use case (https://github.com/microsoft/git/issues/970),
> > N equals 37,815 and caused a slow-down of a simple `git rev-parse
> > --short HEAD` (which is regularly executed as part of `GIT_PS1`) from
> > 0.4s to 4.5s. In another, heavily exercised CI scenario, clone times
> > increased from under 2 minutes to over half an hour.
> 
> Face with Rolling Eyes (1f644) 🙄
> 
> As we grow older, more and more extreme use cases that we initially
> thought were simply crazy become reality.

I have to take back the claim about the clone time, the hunt for that CI
regression is still ongoing, and this patch does _not_ fix it.

Ciao,
Johannes

> 
> > Let's fix this by establishing a fast path for known-new packfiles.
> 
> As long as the caller reliably knows that the pack it has is new and
> cannot be on the list, there is no reason to cycle through all the
> packs in the ring to attempt removing it in vain.
> 
> Clever and clean.
> 
> > diff --git a/packfile.c b/packfile.c
> > index 0eee45055f..f80f05a1fe 100644
> > --- a/packfile.c
> > +++ b/packfile.c
> > @@ -781,7 +781,7 @@ void packfile_store_add_pack(struct odb_source_packed *store,
> >  	if (pack->pack_fd != -1)
> >  		pack_open_fds++;
> >  
> > -	packfile_list_append(&store->packs, pack);
> > +	packfile_list_append(&store->packs, pack, 1);
> >  	strmap_put(&store->packs_by_path, pack->pack_name, pack);
> >  }
> >  
> > diff --git a/t/perf/p5303-many-packs.sh b/t/perf/p5303-many-packs.sh
> > index af173a7b73..4221f9dd70 100755
> > --- a/t/perf/p5303-many-packs.sh
> > +++ b/t/perf/p5303-many-packs.sh
> > @@ -141,4 +141,8 @@ test_perf "load 10,000 packs" '
> >  	git rev-parse --verify "HEAD^{commit}"
> >  '
> >  
> > +test_perf "abbreviate with 10,000 packs" '
> > +	git rev-parse --short HEAD
> > +'
> > +
> >  test_done
> >
> > base-commit: 11c6700f10234578d10523faf35656ca491425c9
>