From: Patrick Steinhardt <ps@pks.im>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, "Jeff King" <peff@peff.net>,
"Felipe Contreras" <felipe.contreras@gmail.com>,
"SZEDER Gábor" <szeder.dev@gmail.com>,
"Chris Torek" <chris.torek@gmail.com>,
"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Taylor Blau" <me@ttaylorr.com>
Subject: Re: [PATCH v3 3/4] revision: avoid loading object headers multiple times
Date: Tue, 3 Aug 2021 11:07:29 +0200 [thread overview]
Message-ID: <YQkHUZKZ6gkFkaD0@ncase> (raw)
In-Reply-To: <xmqq5ywnws6v.fsf@gitster.g>
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On Mon, Aug 02, 2021 at 12:40:56PM -0700, Junio C Hamano wrote:
> Patrick Steinhardt <ps@pks.im> writes:
>
> > When loading references, we try to optimize loading of commits by using
> > the commit graph. To do so, we first need to determine whether the
> > object actually is a commit or not, which is why we always execute
> > `oid_object_info()` first. Like this, we'll unpack the object header of
> > each object first.
> >
> > This pattern can be quite inefficient in case many references point to
> > the same commit: if the object didn't end up in the cached objects, then
> > we'll repeatedly unpack the same object header, even if we've already
> > seen the object before.
> > ...
> > Assuming that in almost all repositories, most references will point to
> > either a tag or a commit, we'd have a modest increase in memory
> > consumption of about 12.5% here.
>
> I wonder if we can also say almost all repositories, the majority of
> refs point at the same object. If that holds, this would certainly
> be a win, but otherwise, it is not so clear.
I doubt that's the case in general. I rather assume that it's typically
going to be a smallish subset that points to the same commit, but for
these cases we at least avoid doing the lookup multiple times. As I
said, it's definitely a tradeoff between memory and performance: in the
worst case (all references point to different blobs) we allocate 33%
more memory without having any speedups. A more realistic scenario would
probably be something like a trunk-based development repo, where there's
a single branch only and the rest is tags. There we'd allocate 11% more
memory without any speedups. In general, it's going to be various shades
of gray, where we allocate something from 0% to 11% more memory while
getting some modest speedups in some cases.
So if we only inspect this commit as a standalone it's definitely
debatable whether we'd want to take it or not. But one important thing
is that it's a prerequisite for patch 4/4: in order to not parse commits
in case they're part of the commit-graph, we need to first obtain an
object such that we can fill it in via the graph. So we have to call
`lookup_unknown_object()` anyway. Might be sensible to document this as
part of the commit message.
Patrick
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next prev parent reply other threads:[~2021-08-03 9:07 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-28 5:33 [PATCH v2 0/3] Speed up connectivity checks via bitmaps Patrick Steinhardt
2021-06-28 5:33 ` [PATCH v2 1/3] p5400: add perf tests for git-receive-pack(1) Patrick Steinhardt
2021-06-28 7:49 ` Ævar Arnfjörð Bjarmason
2021-06-29 6:18 ` Patrick Steinhardt
2021-06-29 12:09 ` Ævar Arnfjörð Bjarmason
2021-06-28 5:33 ` [PATCH v2 2/3] receive-pack: skip connectivity checks on delete-only commands Patrick Steinhardt
2021-06-28 8:00 ` Ævar Arnfjörð Bjarmason
2021-06-28 8:06 ` Ævar Arnfjörð Bjarmason
2021-06-29 6:26 ` Patrick Steinhardt
2021-06-30 1:31 ` Jeff King
2021-06-30 1:35 ` Jeff King
2021-06-30 13:52 ` Patrick Steinhardt
2021-06-28 5:33 ` [PATCH v2 3/3] connected: implement connectivity check using bitmaps Patrick Steinhardt
2021-06-28 20:23 ` Taylor Blau
2021-06-29 22:44 ` Taylor Blau
2021-06-30 2:04 ` Jeff King
2021-06-30 3:07 ` Taylor Blau
2021-06-30 5:45 ` Jeff King
2021-07-02 17:44 ` Taylor Blau
2021-07-02 21:21 ` Jeff King
2021-06-30 1:51 ` Jeff King
2021-07-20 14:26 ` Patrick Steinhardt
2021-08-02 9:37 ` [PATCH v3 0/4] Speed up connectivity checks Patrick Steinhardt
2021-08-02 9:38 ` [PATCH v3 1/4] connected: do not sort input revisions Patrick Steinhardt
2021-08-02 12:49 ` Ævar Arnfjörð Bjarmason
2021-08-03 8:50 ` Patrick Steinhardt
2021-08-04 11:01 ` Ævar Arnfjörð Bjarmason
2021-08-02 19:00 ` Junio C Hamano
2021-08-03 8:55 ` Patrick Steinhardt
2021-08-03 21:47 ` Junio C Hamano
2021-08-02 9:38 ` [PATCH v3 2/4] revision: stop retrieving reference twice Patrick Steinhardt
2021-08-02 12:53 ` Ævar Arnfjörð Bjarmason
2021-08-02 9:38 ` [PATCH v3 3/4] revision: avoid loading object headers multiple times Patrick Steinhardt
2021-08-02 12:55 ` Ævar Arnfjörð Bjarmason
2021-08-05 10:12 ` Patrick Steinhardt
2021-08-02 19:40 ` Junio C Hamano
2021-08-03 9:07 ` Patrick Steinhardt [this message]
2021-08-06 14:17 ` Patrick Steinhardt
2021-08-02 9:38 ` [PATCH v3 4/4] revision: avoid hitting packfiles when commits are in commit-graph Patrick Steinhardt
2021-08-02 20:01 ` Junio C Hamano
2021-08-03 9:16 ` Patrick Steinhardt
2021-08-03 21:56 ` Junio C Hamano
2021-08-05 11:01 ` Patrick Steinhardt
2021-08-05 16:16 ` Junio C Hamano
2021-08-04 10:51 ` Ævar Arnfjörð Bjarmason
2021-08-05 11:25 ` [PATCH v4 0/6] Speed up connectivity checks Patrick Steinhardt
2021-08-05 11:25 ` [PATCH v4 1/6] revision: separate walk and unsorted flags Patrick Steinhardt
2021-08-05 18:47 ` Junio C Hamano
2021-08-05 11:25 ` [PATCH v4 2/6] connected: do not sort input revisions Patrick Steinhardt
2021-08-05 18:44 ` Junio C Hamano
2021-08-06 6:00 ` Patrick Steinhardt
2021-08-06 16:50 ` Junio C Hamano
2021-08-05 11:25 ` [PATCH v4 3/6] revision: stop retrieving reference twice Patrick Steinhardt
2021-08-05 11:25 ` [PATCH v4 4/6] revision: avoid loading object headers multiple times Patrick Steinhardt
2021-08-05 11:25 ` [PATCH v4 5/6] commit-graph: split out function to search commit position Patrick Steinhardt
2021-08-05 11:25 ` [PATCH v4 6/6] revision: avoid hitting packfiles when commits are in commit-graph Patrick Steinhardt
2021-08-09 8:00 ` [PATCH v5 0/5] Speed up connectivity checks Patrick Steinhardt
2021-08-09 8:02 ` Patrick Steinhardt
2021-08-09 8:11 ` Patrick Steinhardt
2021-08-09 8:11 ` [PATCH v5 1/5] revision: separate walk and unsorted flags Patrick Steinhardt
2021-08-09 8:11 ` [PATCH v5 2/5] connected: do not sort input revisions Patrick Steinhardt
2021-08-09 8:11 ` [PATCH v5 3/5] revision: stop retrieving reference twice Patrick Steinhardt
2021-08-09 8:11 ` [PATCH v5 4/5] commit-graph: split out function to search commit position Patrick Steinhardt
2021-08-09 8:12 ` [PATCH v5 5/5] revision: avoid hitting packfiles when commits are in commit-graph Patrick Steinhardt
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