From: Thomas Rast <trast@inf.ethz.ch>
To: Jeff King <peff@peff.net>
Cc: git@vger.kernel.org, "Junio C Hamano" <gitster@pobox.com>,
"Ramkumar Ramachandra" <artagnon@gmail.com>,
"Alex Bennée" <kernel-hacker@bennee.com>,
"Antoine Pelisse" <apelisse@gmail.com>,
"John Keeping" <john@keeping.me.uk>,
"Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>
Subject: Re: [PATCH 2/2] lookup_commit_reference_gently: do not read non-{tag,commit}
Date: Fri, 31 May 2013 10:08:06 +0200 [thread overview]
Message-ID: <87sj138tcp.fsf@linux-k42r.v.cablecom.net> (raw)
In-Reply-To: <20130530212223.GA2135@sigill.intra.peff.net> (Jeff King's message of "Thu, 30 May 2013 17:22:23 -0400")
Jeff King <peff@peff.net> writes:
> On Thu, May 30, 2013 at 10:00:23PM +0200, Thomas Rast wrote:
>
>> lookup_commit_reference_gently unconditionally parses the object given
>> to it. This slows down git-describe a lot if you have a repository
>> with large tagged blobs in it: parse_object() will read the entire
>> blob and verify that its sha1 matches, only to then throw it away.
>>
>> Speed it up by checking the type with sha1_object_info() prior to
>> unpacking.
>
> This would speed up the case where we do not end up looking at the
> object at all, but it will slow down the (presumably common) case where
> we will in fact find a commit and end up parsing the object anyway.
>
> Have you measured the impact of this on normal operations? During a
> traversal, we spend a measurable amount of time looking up commits in
> packfiles, and this would presumably double it.
I don't think so, but admittedly I didn't measure it.
The reason why it's unlikely is that this is specific to
lookup_commit_reference_gently, which according to some grepping is
usually done on refs or values that refs might have; e.g. on the old&new
sides of a fetch in remote.c, or in many places in the callback of some
variant of for_each_ref.
Of course if you have a ridiculously large number of refs (and I gather
_you_ do), this will hurt somewhat in the usual case, but speed up the
case where there is a ref (usually a lightweight tag) directly pointing
at a large blob.
I'm not sure this can be fixed without the change you outline here:
> This is not the first time I have seen this tradeoff in git. It would
> be nice if our object access was structured to do incremental
> examination of the objects (i.e., store the packfile index lookup or
> partial unpack of a loose object header, and then use that to complete
> the next step of actually getting the contents).
But in any case I see the point, I should try and gather some
performance numbers.
--
Thomas Rast
trast@{inf,student}.ethz.ch
next prev parent reply other threads:[~2013-05-31 8:08 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-30 10:38 Poor performance of git describe in big repos Alex Bennée
2013-05-30 11:33 ` Ramkumar Ramachandra
2013-05-30 13:09 ` Alex Bennée
2013-05-30 14:32 ` Ramkumar Ramachandra
2013-05-30 15:01 ` Alex Bennée
2013-05-30 15:17 ` Ramkumar Ramachandra
2013-05-30 15:33 ` Thomas Rast
2013-05-30 16:01 ` Alex Bennée
2013-05-30 16:21 ` Thomas Rast
2013-05-30 16:44 ` Thomas Rast
2013-05-30 19:01 ` Antoine Pelisse
2013-05-30 20:00 ` [PATCH 1/2] sha1_file: silence sha1_loose_object_info Thomas Rast
2013-05-30 20:00 ` [PATCH 2/2] lookup_commit_reference_gently: do not read non-{tag,commit} Thomas Rast
2013-05-30 21:22 ` Jeff King
2013-05-31 0:52 ` Duy Nguyen
2013-05-31 8:08 ` Thomas Rast [this message]
2013-05-31 16:00 ` Jeff King
2013-05-31 6:43 ` Ramkumar Ramachandra
2013-05-31 8:16 ` Thomas Rast
2013-05-30 19:30 ` Poor performance of git describe in big repos John Keeping
2013-05-31 8:14 ` Alex Bennée
2013-05-31 8:24 ` Thomas Rast
2013-05-31 8:40 ` Alex Bennée
2013-05-31 8:46 ` Thomas Rast
2013-05-31 9:57 ` Alex Bennée
2013-06-03 8:02 ` Alex Bennée
2013-06-03 16:32 ` Junio C Hamano
2013-06-03 17:48 ` Junio C Hamano
2013-05-31 10:27 ` Thomas Rast
2013-05-31 16:17 ` Jeff King
2013-06-03 8:39 ` Alex Bennée
2013-06-03 14:49 ` Jeff King
2013-05-31 8:32 ` John Keeping
2013-05-31 8:49 ` Alex Bennée
2013-05-31 8:59 ` John Keeping
2013-05-30 11:48 ` John Keeping
2013-05-30 12:29 ` Alex Bennée
2013-05-30 13:20 ` Duy Nguyen
[not found] ` <CAJ-05NPacjAEC99Ntd9eMnTD9_PMMYFob-_tAx5CeSB79TkRSg@mail.gmail.com>
2013-05-30 13:45 ` Duy Nguyen
2013-05-30 14:02 ` Alex Bennée
2013-05-30 13:16 ` Alex Bennée
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