netdev - Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

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Date:	Mon, 28 Oct 2013 17:20:45 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, sebastien.dugue@...l.net,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's


* Neil Horman <nhorman@...driver.com> wrote:

> Base:
>        0.093269042 seconds time elapsed                                          ( +-  2.24% )
> Prefetch (5x64):
>        0.079440009 seconds time elapsed                                          ( +-  2.29% )
> Parallel ALU:
>        0.087666677 seconds time elapsed                                          ( +-  4.01% )
> Prefetch + Parallel ALU:
>        0.080758702 seconds time elapsed                                          ( +-  2.34% )
> 
> So we can see here that we get about a 1% speedup between the base 
> and the both (Prefetch + Parallel ALU) case, with prefetch 
> accounting for most of that speedup.

Hm, there's still something strange about these results. So the 
range of the results is 790-930 nsecs. The noise of the measurements 
is 2%-4%, i.e. 20-40 nsecs.

The prefetch-only result itself is the fastest of all - 
statistically equivalent to the prefetch+parallel-ALU result, within 
the noise range.

So if prefetch is enabled, turning on parallel-ALU has no measurable 
effect - which is counter-intuitive. Do you have an 
theory/explanation for that?

Thanks,

	Ingo
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