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

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Date:	Fri, 08 Nov 2013 11:33:13 -0800
From:	Joe Perches <joe@...ches.com>
To:	Neil Horman <nhorman@...driver.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 Fri, 2013-11-08 at 14:01 -0500, Neil Horman wrote:
> 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:

Thanks.

Reformatting the table so it's a bit more
readable/comparable for me:

len	SetSz	Loops	cycles/byte
			limited	unlimited
1500B	64MB	1M	1.3442	1.3605
1500B	128MB	1M	1.3410	1.3542
1500B	256MB	1M	1.3536	1.3710
1500B	512MB	1M	1.3463	1.3536
9000B	64MB	1M	0.8522	0.8504
9000B	128MB	1M	0.8528	0.8536
9000B	256MB	1M	0.8532	0.8520
9000B	512MB	1M	0.8527	0.8525
64KB	64MB	1M	0.7686	0.7683
64KB	128MB	1M	0.7695	0.7686
64KB	256MB	1M	0.7699	0.7708
64KB	512MB	1M	0.7799	0.7694

This data appears to show some value
in capping for 1500b lengths and noise
for shorter and longer lengths.

Any idea what the actual distribution of
do_csum lengths is under various loads?


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