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

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Message-ID: <20131013125344.GC9286@neilslaptop.think-freely.org>
Date:	Sun, 13 Oct 2013 08:53:44 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, sebastien.dugue@...l.net,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

On Sat, Oct 12, 2013 at 03:29:24PM -0700, H. Peter Anvin wrote:
> On 10/11/2013 09:51 AM, Neil Horman 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
> 
> On hardware that implement ADCX/ADOX then you should also be able to
> have additional streams interleaved since those instructions allow for
> dual carry chains.
> 
Ok, thats a good idea, I'll look into those instructions this week
Neil

> 	-hpa
> 
> 
> 
> 
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