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

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Message-ID: <20131013125302.GB9286@neilslaptop.think-freely.org>
Date:	Sun, 13 Oct 2013 08:53:03 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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] x86: Run checksumming in parallel accross multiple alu's

On Sat, Oct 12, 2013 at 07:21:24PM +0200, Ingo Molnar wrote:
> 
> * Neil Horman <nhorman@...driver.com> wrote:
> 
> > Sébastien Dugué reported to me that devices implementing ipoib (which 
> > don't have checksum offload hardware were spending a significant amount 
> > of time computing checksums.  We found that by splitting the checksum 
> > computation into two separate streams, each skipping successive elements 
> > of the buffer being summed, we could parallelize the checksum operation 
> > accros multiple alus.  Since neither chain is dependent on the result of 
> > the other, we get a speedup in execution (on hardware that has multiple 
> > alu's available, which is almost ubiquitous on x86), and only a 
> > negligible decrease on hardware that has only a single alu (an extra 
> > addition is introduced).  Since addition in commutative, the result is 
> > the same, only faster
> 
> This patch should really come with measurement numbers: what performance 
> increase (and drop) did you get on what CPUs.
> 
> Thanks,
> 
Sure, I can gather some stats for you.  I'll post them later this week
Neil

> 	Ingo
> 
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