From: Jonathan Nieder <jrnieder@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: git@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Peter Zijlstra" <a.p.zijlstra@chello.nl>,
"Arnaldo Carvalho de Melo" <acme@redhat.com>,
"Frédéric Weisbecker" <fweisbec@gmail.com>,
"Pekka Enberg" <penberg@cs.helsinki.fi>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 04:31:21 -0500 [thread overview]
Message-ID: <20110428093120.GA377@elie> (raw)
In-Reply-To: <20110428063625.GB952@elte.hu>
Ingo Molnar wrote:
> * Jonathan Nieder <jrnieder@gmail.com> wrote:
>> E.g., how would something like
>>
>> const unsigned int *start1 = (const unsigned int *) sha1;
>> const unsigned int *start2 = (const unsigned int *) sha2;
>>
>> if (likely(*start1 != *start2)) {
>> if (*start1 < *start2)
>> return -1;
>> return +1;
>> }
>> return memcmp(sha1 + 4, sha2 + 4, 16);
>>
>> perform?
>
> Note that this function wont work on like 99% of the systems out there due to
> endianness assumptions in Git.
Yes, I was greedy and broke the semantics, and my suggestion was
nonsensical for other reasons (e.g., alignment), too. I should have
written something like:
if (likely(*sha1 != *sha2)) {
if (*sha1 < *sha2)
return -1;
return +1;
}
return memcmp(sha1, sha2, 20);
since speeding it up 255/256 times seems good enough already.
> Also, your hypothetical smart compiler would recognize the above as equivalent
> to memcmp(sha1, sha2, 20) and could rewrite it again - so we'd be back to
> square 1.
True. The real point is a "likely" to explain to human readers what
is happening.
> Having said that, it would be nice if someone could test these two patches on a
> modern AMD box, using the perf stat from here:
>
> http://people.redhat.com/mingo/tip.git/README
>
> cd tools/perf/
> make -j install
>
> and do something like this to test git gc's performance:
>
> $ perf stat --sync --repeat 10 ./git gc
>
> ... to see whether these speedups are generic, or somehow Intel CPU specific.
Sounds like fun. Will try to find time to play around with this in
the next few days.
> Well i messed up endianness in an early version of this patch and 'git gc' was
> eminently unhappy about it! I have not figured out which part of Git relies on
> the comparison result though - most places seem to use the result as a boolean.
I think hashcmp is used to run binary searches within a packfile
index. Thanks for explaining.
Regards,
Jonathan
next prev parent reply other threads:[~2011-04-28 9:31 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-27 22:51 [PATCH] git gc: Speed it up by 18% via faster hash comparisons Ingo Molnar
2011-04-27 23:10 ` Ingo Molnar
2011-04-27 23:18 ` Jonathan Nieder
2011-04-28 6:36 ` Ingo Molnar
2011-04-28 9:31 ` Jonathan Nieder [this message]
2011-04-28 10:36 ` Ingo Molnar
2011-04-28 9:32 ` Dmitry Potapov
2011-04-27 23:32 ` Junio C Hamano
2011-04-28 0:35 ` Ralf Baechle
2011-04-28 8:18 ` Bernhard R. Link
2011-04-28 9:42 ` Andreas Ericsson
2011-04-28 9:55 ` Erik Faye-Lund
2011-04-28 20:19 ` H. Peter Anvin
2011-04-28 6:27 ` Ingo Molnar
2011-04-28 9:17 ` Erik Faye-Lund
2011-04-28 9:33 ` Ingo Molnar
2011-04-28 9:37 ` Ingo Molnar
2011-04-28 9:50 ` Erik Faye-Lund
2011-04-28 10:10 ` Pekka Enberg
2011-04-28 10:19 ` Erik Faye-Lund
2011-04-28 10:30 ` Pekka Enberg
2011-04-28 11:59 ` Erik Faye-Lund
2011-04-28 12:12 ` Pekka Enberg
2011-04-28 12:36 ` Jonathan Nieder
2011-04-28 12:40 ` Erik Faye-Lund
2011-04-28 13:37 ` Ingo Molnar
2011-04-28 15:14 ` Ingo Molnar
2011-04-28 16:00 ` Erik Faye-Lund
2011-04-28 20:32 ` Ingo Molnar
2011-04-29 7:05 ` Alex Riesen
2011-04-29 16:24 ` H. Peter Anvin
2011-04-28 12:16 ` Tor Arntsen
2011-04-28 20:23 ` H. Peter Anvin
2011-04-28 12:17 ` Andreas Ericsson
2011-04-28 12:28 ` Erik Faye-Lund
2011-04-28 10:19 ` Ingo Molnar
2011-04-28 12:02 ` Nguyen Thai Ngoc Duy
2011-04-28 12:18 ` Erik Faye-Lund
2011-04-28 20:20 ` Junio C Hamano
2011-04-28 16:36 ` Dmitry Potapov
2011-04-28 8:52 ` Dmitry Potapov
2011-04-28 9:11 ` Ingo Molnar
2011-04-28 9:31 ` Dmitry Potapov
2011-04-28 9:44 ` Ingo Molnar
2011-04-28 9:38 ` Ingo Molnar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20110428093120.GA377@elie \
--to=jrnieder@gmail.com \
--cc=a.p.zijlstra@chello.nl \
--cc=acme@redhat.com \
--cc=fweisbec@gmail.com \
--cc=git@vger.kernel.org \
--cc=hpa@zytor.com \
--cc=mingo@elte.hu \
--cc=penberg@cs.helsinki.fi \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).