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

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Date:	Fri, 01 Nov 2013 13:26:52 -0700
From:	Joe Perches <joe@...ches.com>
To:	Neil Horman <nhorman@...driver.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, 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.

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