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

[Junio C Hamano <gitster@pobox.com>]

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
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.

> 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