netdev - RE: [Fwd: Re: [PATCH v2 2/2] x86: add prefetching to do

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B740E@saturn3.aculab.com>
Date:	Wed, 13 Nov 2013 10:09:51 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Neil Horman" <nhorman@...driver.com>,
	"Joe Perches" <joe@...ches.com>
Cc:	"netdev" <netdev@...r.kernel.org>, "Dave Jones" <davej@...hat.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>, "Eric Dumazet" <eric.dumazet@...il.com>
Subject: RE: [Fwd: Re: [PATCH v2 2/2] x86: add prefetching to do_csum]

> Sure, I modified the code so that we only prefetched 2 cache lines ahead, but
> only if the overall length of the input buffer is more than 2 cache lines.
> Below are the results (all counts are the average of 1000000 iterations of the
> csum operation, as previous tests were, I just omitted that column).

Hmmm.... averaging over 100000 iterations means that all the code
is in the i-cache and the branch predictor will be correctly primed.

For short checksum requests I'd guess that the relevant data
has just been written and is already in the cpu cache (unless
there has been a process and cpu switch).
So prefetch is likely to be unnecessary.

If you assume that the checksum code isn't in the i-cache then
small requests are likely to be dominated by the code size.

	David



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