Re: [PATCH v2] fetch-pack: don't try to fetch peel values with --all

From: Kirill Smelkov <kirr@nexedi.com>
To: Jeff King <peff@peff.net>
Cc: Eric Sunshine <sunshine@sunshineco.com>,
	Junio C Hamano <gitster@pobox.com>,
	Jonathan Tan <jonathantanmy@google.com>,
	Brandon Williams <bmwill@google.com>,
	Takuto Ikuta <tikuta@chromium.org>,
	Jeff Hostetler <jeffhost@microsoft.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Michael Haggerty <mhagger@alum.mit.edu>,
	Git List <git@vger.kernel.org>
Subject: Re: [PATCH v2] fetch-pack: don't try to fetch peel values with --all
Date: Thu, 14 Jun 2018 05:29:42 +0000	[thread overview]
Message-ID: <20180614052938.GB2861@deco.navytux.spb.ru> (raw)
In-Reply-To: <20180613211302.GB15574@sigill.intra.peff.net>

On Wed, Jun 13, 2018 at 05:13:02PM -0400, Jeff King wrote:
> On Tue, Jun 12, 2018 at 06:54:17PM +0000, Kirill Smelkov wrote:
> 
> > > If an extra connection isn't a problem, you might be better off with
> > > "git ls-remote", and then picking through the results for refs of
> > > interest, and then "git fetch-pack" to actually get the pack. That's how
> > > git-fetch worked when it was a shell script (e.g., see c3a200120d, the
> > > last shell version).
> > 
> > Yes, this is what I ended up doing:
> > 
> > https://lab.nexedi.com/kirr/git-backup/commit/899103bf
> > 
> > but for another reason - to avoid repeating for every fetched repository
> > slow (in case of my "big" destination backup repository) quickfetch()
> > checking in every spawned `git fetch`: git-backup can build index of
> > objects we already have ourselves only once at startup, and then in
> > fetch, after checking lsremote output, consult that index, and if we see
> > we already have everything for an advertised reference - just avoid
> > giving it to fetch-pack to process. It turns out for many pulled
> > repositories there is usually no references changed at all and this way
> > fetch-pack can be skipped completely:
> > 
> > https://lab.nexedi.com/kirr/git-backup/commit/3efed898
> 
> Thanks for sharing that, it's an interesting case. I'd hope that
> git-fetch is smart enough not to even bother with quickfetch() if there
> are no refs to update. But if we have even one change to fetch, then
> yeah, in the general case it makes sense to me that you could do better
> by amortizing the scan of local objects across many operations.

Thanks for feedback. For the reference in case of git-backup `git fetch`
or `git fetch-pack` would have to always do quickfetch scan or equivalent,
because in case of backup repo there is only one reference in it - its
master - and references of backed up repositories do not have anything
representing them in backup.git/refs/ .

Kirill