linux-kernel - Re: x86 memcpy performance

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Message-ID: <6296.1313462075@turing-police.cc.vt.edu>
Date:	Mon, 15 Aug 2011 22:34:35 -0400
From:	Valdis.Kletnieks@...edu
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...e.hu>, melwyn lobo <linux.melwyn@...il.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Sun, 14 Aug 2011 11:59:10 +0200, Borislav Petkov said:

> Benchmarking with 10000 iterations, average results:
> size    XM              MM              speedup
> 119     540.58          449.491         0.8314969419

> 12273   2307.86         4042.88         1.751787902
> 13924   2431.8          4224.48         1.737184756
> 14335   2469.4          4218.82         1.708440514
> 15018   2675.67         1904.07         0.711622886
> 16374   2989.75         5296.26         1.771470902
> 24564   4262.15         7696.86         1.805863077
> 27852   4362.53         3347.72         0.7673805572
> 28672   5122.8          7113.14         1.388524413
> 30033   4874.62         8740.04         1.792967931

The numbers for 15018 and 27852 are *way* odd for the MM case. I don't feel
really good about this till we understand what happened for those two cases.

Also, anytime I see "10000 iterations",  I ask myself if the benchmark rigging
took proper note of hot/cold cache issues.  That *may* explain the two oddball
results we see above - but not knowing more about how it was benched, it's hard
to say.


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