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

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Message-ID: <20131108190136.GB16052@hmsreliant.think-freely.org>
Date:	Fri, 8 Nov 2013 14:01:36 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Joe Perches <joe@...ches.com>
Cc:	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
Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum

On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
> On Wed, 2013-11-06 at 10:54 -0500, Neil Horman wrote:
> > On Wed, Nov 06, 2013 at 10:34:29AM -0500, Dave Jones wrote:
> > > On Wed, Nov 06, 2013 at 10:23:19AM -0500, Neil Horman wrote:
> > >  > do_csum was identified via perf recently as a hot spot when doing
> > >  > receive on ip over infiniband workloads.  After alot of testing and
> > >  > ideas, we found the best optimization available to us currently is to
> > >  > prefetch the entire data buffer prior to doing the checksum
> []
> > I'll fix this up and send a v3, but I'll give it a day in case there are more
> > comments first.
> 
> Perhaps a reduction in prefetch loop count helps.
> 
> Was capping the amount prefetched and letting the
> hardware prefetch also tested?
> 
> 	prefetch_lines(buff, min(len, cache_line_size() * 8u));
> 

Just tested this out:

With limiting:
1500B   | 64MB  | 1000000       | 1.344167
1500B   | 128MB | 1000000       | 1.340970
1500B   | 256MB | 1000000       | 1.353562
1500B   | 512MB | 1000000       | 1.346349
9000B   | 64MB  | 1000000       | 0.852174
9000B   | 128MB | 1000000       | 0.852765
9000B   | 256MB | 1000000       | 0.853153
9000B   | 512MB | 1000000       | 0.852661
64KB    | 64MB  | 1000000       | 0.768585
64KB    | 128MB | 1000000       | 0.769465
64KB    | 256MB | 1000000       | 0.769909
64KB    | 512MB | 1000000       | 0.779895


Without limiting

1500B   | 64MB  | 1000000       | 1.360525
1500B   | 128MB | 1000000       | 1.354220
1500B   | 256MB | 1000000       | 1.371037
1500B   | 512MB | 1000000       | 1.353557
9000B   | 64MB  | 1000000       | 0.850415
9000B   | 128MB | 1000000       | 0.853642
9000B   | 256MB | 1000000       | 0.852048
9000B   | 512MB | 1000000       | 0.852484
64KB    | 64MB  | 1000000       | 0.768261
64KB    | 128MB | 1000000       | 0.768566
64KB    | 256MB | 1000000       | 0.770822
64KB    | 512MB | 1000000       | 0.769391

Doesn't look like much consistent improvement.

Neil

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