From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 18 Jun 2025 21:50:49 GMT
Subject: Re: [RFC PATCH 1/2] fetch-prune: optimize dangling-ref reporting
Message-ID: <xmqqzfe4d8hy.fsf@gitster.g>
In-Reply-To: <20250618211024.2332525-2-phil.hord@gmail.com>

Phil Hord <phil.hord@gmail.com> writes:

> From: Phil Hord <phil.hord@gmail.com>
>
> When pruning during `git fetch` we check each pruned ref against the
> ref_store one at a time to decide whether to report it as dangling.
> This causes every local ref to be scanned for each ref being pruned.
>
> If there are N refs in the repo and M refs being pruned, this code is
> O(M*N). However, `git remote prune` uses a very similar function that
> is only O(N*log(M)).
>
> Remove the wasteful ref scanning for each pruned ref and use the faster
> version already available in refs_warn_dangling_symrefs.
>
> In a repo with 126,000 refs, where I was pruning 28,000 refs, this
> code made about 3.6 billion calls to strcmp and consumed 410 seconds
> of CPU. (Invariably in that time, my remote would timeout and the
> fetch would fail anyway.)
>
> After this change, the same operation completes in under 4 seconds.

Nice.