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

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Message-ID: <5259CD44.2000200@zytor.com>
Date:	Sat, 12 Oct 2013 15:29:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Neil Horman <nhorman@...driver.com>, linux-kernel@...r.kernel.org
CC:	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 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.

	-hpa



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