Message-ID: <5259CD44.2000200@zytor.com>
Date: Sat, 12 Oct 2013 15:29:24 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Neil Horman <nhorman@tuxdriver.com>, linux-kernel@vger.kernel.org
CC: sebastien.dugue@bull.net, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's
References: <1381510298-20572-1-git-send-email-nhorman@tuxdriver.com>
In-Reply-To: <1381510298-20572-1-git-send-email-nhorman@tuxdriver.com>
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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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