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

[Neil Horman]

On Sat, Oct 12, 2013 at 07:21:24PM +0200, Ingo Molnar wrote:
> 
> * Neil Horman <nhorman@xxxxxxxxxxxxx> 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
> 
> This patch should really come with measurement numbers: what performance 
> increase (and drop) did you get on what CPUs.
> 
> Thanks,
> 
> 	Ingo
> 


So, early testing results today.  I wrote a test module that, allocated a 4k
buffer, initalized it with random data, and called csum_partial on it 100000
times, recording the time at the start and end of that loop.  Results on a 2.4
GHz Intel Xeon processor:

Without patch: Average execute time for csum_partial was 808 ns
With patch: Average execute time for csum_partial was 438 ns


I'm looking into hpa's suggestion to use alternate instructions where available
right now.  I'll have more soon
Neil

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