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From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: Brandon Casey <drafnel@gmail.com>,
	Brandon Casey <bcasey@nvidia.com>,
	"git@vger.kernel.org" <git@vger.kernel.org>,
	Martin Fick <mfick@codeaurora.org>
Subject: Re: [PATCH] remote.c: avoid O(n^2) behavior in match_push_refs by using string_list
Date: Wed, 3 Jul 2013 15:00:47 -0400	[thread overview]
Message-ID: <20130703190047.GA349@sigill.intra.peff.net> (raw)
In-Reply-To: <7vhagbfpwz.fsf@alter.siamese.dyndns.org>

On Wed, Jul 03, 2013 at 11:40:12AM -0700, Junio C Hamano wrote:

> Brandon Casey <drafnel@gmail.com> writes:
> 
> > Right.  For repos with few refs on either side, I don't think there
> > will be any measurable difference.  When pushing a single ref to a
> > repo with a very large number of refs, we will see a very small net
> > loss for the time required to prepare the string list (which grows
> > linearly with the number of remote refs).  After 2 or 3 refs, we
> > should see a net gain.
> >
> > So we're really just improving our worst case performance here.
> 
> ... by penalizing the common case by how much?  If it is not too
> much, then this obviously would be a good change.

I don't think by much. If we have "m" local refs to push and "n" remote
refs, right now we do O(m*n) work ("m" linear searches of the remote
namespace). With Brandon's patch, we do O(n log n) to build the index,
plus O(m log n) for lookups.

So our break-even point is basically m = log n, and for m smaller than
that, we do more work building the index. Your absolute biggest
difference would be pushing a single ref to a repository with a very
large number of refs.

Here are the timings before and after Brandon's patch for pushing a
no-op single ref from a normal repo to one with 370K refs (the same
pathological repo from the upload-pack tests). Times are
best-of-five.

             before     after
     real    0m1.087s   0m1.156s
     user    0m1.344s   0m1.412s
     sys     0m0.288s   0m0.284s

So it's measurable, but even on a pathological worst-case, we're talking
about 6% slowdown.

You could try to guess about when to build the index based on the size
of "m" and "n", but I suspect you'd waste more time calculating whether
to build the index than you would simply building it in most cases.

-Peff

  reply	other threads:[~2013-07-03 19:00 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-07-02 23:53 [PATCH] remote.c: avoid O(n^2) behavior in match_push_refs by using string_list Brandon Casey
2013-07-03  6:23 ` Jeff King
2013-07-03 18:12   ` Brandon Casey
2013-07-03 18:40     ` Junio C Hamano
2013-07-03 19:00       ` Jeff King [this message]
2013-07-03 20:05         ` Brandon Casey
2013-07-03 19:21       ` Brandon Casey
2013-07-03 20:22         ` Junio C Hamano
2013-07-08  7:02           ` [PATCH v2] remote.c: avoid O(m*n) behavior in match_push_refs Brandon Casey
2013-07-08  7:50             ` Jeff King
2013-07-08  8:58               ` [PATCH v2 w/prune index] " Brandon Casey
2013-07-08 16:12                 ` Junio C Hamano

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