linux-kernel - Re: [Fwd: Re: [PATCH v2 2/2] x86: add prefetching to do

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Message-ID: <20131113123010.GB2993@hmsreliant.think-freely.org>
Date:	Wed, 13 Nov 2013 07:30:10 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Joe Perches <joe@...ches.com>, 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]

On Wed, Nov 13, 2013 at 10:09:51AM -0000, David Laight wrote:
> > 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.
> 
I'm not sure, whats the typical capacity for the branch predictors ability to
remember code paths?  I ask because the most likely use of do_csum will be in
the receive path of the networking stack (specifically in the softirq handler).
So if we run do_csum once, we're likely to run it many more times, as we clean
out an adapters receive queue.

Neil

> 	David
> 
> 
> 
> 
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