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

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Message-ID: <20131031143325.GB25894@hmsreliant.think-freely.org>
Date:	Thu, 31 Oct 2013 10:33:25 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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,
	netdev@...r.kernel.org
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

On Thu, Oct 31, 2013 at 11:22:00AM +0100, Ingo Molnar wrote:
> 
> * Neil Horman <nhorman@...driver.com> wrote:
> 
> > > etc. For such short runtimes make sure the last column displays 
> > > close to 100%, so that the PMU results become trustable.
> > > 
> > > A nehalem+ PMU will allow 2-4 events to be measured in parallel, 
> > > plus generics like 'cycles', 'instructions' can be added 'for free' 
> > > because they get counted in a separate (fixed purpose) PMU register.
> > > 
> > > The last colum tells you what percentage of the runtime that 
> > > particular event was actually active. 100% (or empty last column) 
> > > means it was active all the time.
> > > 
> > > Thanks,
> > > 
> > > 	Ingo
> > > 
> > 
> > Hmm, 
> > 
> > I ran this test:
> > 
> > for i in `seq 0 1 3`
> > do
> > echo $i > /sys/module/csum_test/parameters/module_test_mode
> > taskset -c 0 perf stat --repeat 20 -C 0 -e L1-dcache-load-misses -e L1-dcache-prefetches -e cycles -e instructions -ddd ./test.sh
> > done
> 
> You need to remove '-ddd' which is a shortcut for a ton of useful 
> events, but here you want to use fewer events, to increase the 
> precision of the measurement.
> 
> Thanks,
> 
> 	Ingo
> 

Thank you ingo, that fixed it.  I'm trying some other variants of the csum
algorithm that Doug and I discussed last night, but FWIW, the relative
performance of the 4 test cases (base/prefetch/parallel/both) remains unchanged.
I'm starting to feel like at this point, theres very little point in doing
parallel alu operations (unless we can find a way to break the dependency on the
carry flag, which is what I'm tinkering with now).
Neil

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