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

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Message-ID: <20131102020713.GA16290@hmsreliant.think-freely.org>
Date:	Fri, 1 Nov 2013 22:07:13 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Doug Ledford <dledford@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

On Fri, Nov 01, 2013 at 01:26:52PM -0700, Joe Perches wrote:
> On Fri, 2013-11-01 at 15:58 -0400, Neil Horman wrote:
> > On Fri, Nov 01, 2013 at 12:45:29PM -0700, Joe Perches wrote:
> > > On Fri, 2013-11-01 at 13:37 -0400, Neil Horman wrote:
> > > 
> > > > I think it would be better if we just did the prefetch here
> > > > and re-addressed this area when AVX (or addcx/addox) instructions were available
> > > > for testing on hardware.
> > > 
> > > Could there be a difference if only a single software
> > > prefetch was done at the beginning of transfer before
> > > the while loop and hardware prefetches did the rest?
> > > 
> > I wouldn't think so.  If hardware was going to do any prefetching based on
> > memory access patterns it will do so regardless of the leading prefetch, and
> > that first prefetch isn't helpful because we still wind up stalling on the adds
> > while its completing
> 
> I imagine one benefit to be helping prevent
> prefetching beyond the actual data required.
> 
> Maybe some hardware optimizes prefetch stride
> better than 5*64.
> 
> I wonder also if using
> 
> 	if (count > some_length)
> 		prefetch
> 	while (...)
> 
> helps small lengths more than the test/jump cost.
> 
We've already done this and it is in fact the best performing.  I'll be posting
that patch along with ingos request to add do_csum to the perf bench code when I
have that done
Best
Neil

> 
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