Date: Sun, 13 Oct 2013 08:53:44 -0400
From: Neil Horman <nhorman@tuxdriver.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, 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
Message-ID: <20131013125344.GC9286@neilslaptop.think-freely.org>
References: <1381510298-20572-1-git-send-email-nhorman@tuxdriver.com>
 <5259CD44.2000200@zytor.com>
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On Sat, Oct 12, 2013 at 03:29:24PM -0700, H. Peter Anvin wrote:
> On 10/11/2013 09:51 AM, Neil Horman wrote:
> > S?bastien Dugu? 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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