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SubjectRe: [PATCH v2 2/2] x86: add prefetching to do_csum
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On Fri, 2013-11-08 at 15:14 -0500, Neil Horman wrote:
> On Fri, Nov 08, 2013 at 11:33:13AM -0800, Joe Perches wrote:
> > On Fri, 2013-11-08 at 14:01 -0500, Neil Horman wrote:
> > > On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
> > > > On Wed, 2013-11-06 at 10:54 -0500, Neil Horman wrote:
> > > > > On Wed, Nov 06, 2013 at 10:34:29AM -0500, Dave Jones wrote:
> > > > > > On Wed, Nov 06, 2013 at 10:23:19AM -0500, Neil Horman wrote:
> > > > > > > do_csum was identified via perf recently as a hot spot when doing
> > > > > > > receive on ip over infiniband workloads. After alot of testing and
> > > > > > > ideas, we found the best optimization available to us currently is to
> > > > > > > prefetch the entire data buffer prior to doing the checksum
> > > > []
> > > > > I'll fix this up and send a v3, but I'll give it a day in case there are more
> > > > > comments first.
> > > >
> > > > Perhaps a reduction in prefetch loop count helps.
> > > >
> > > > Was capping the amount prefetched and letting the
> > > > hardware prefetch also tested?
> > > >
> > > > prefetch_lines(buff, min(len, cache_line_size() * 8u));
> > > >
> > >
> > > Just tested this out:
> >
> > Thanks.
> >
> > Reformatting the table so it's a bit more
> > readable/comparable for me:
> >
> > len SetSz Loops cycles/byte
> > limited unlimited
> > 1500B 64MB 1M 1.3442 1.3605
> > 1500B 128MB 1M 1.3410 1.3542
> > 1500B 256MB 1M 1.3536 1.3710
> > 1500B 512MB 1M 1.3463 1.3536
> > 9000B 64MB 1M 0.8522 0.8504
> > 9000B 128MB 1M 0.8528 0.8536
> > 9000B 256MB 1M 0.8532 0.8520
> > 9000B 512MB 1M 0.8527 0.8525
> > 64KB 64MB 1M 0.7686 0.7683
> > 64KB 128MB 1M 0.7695 0.7686
> > 64KB 256MB 1M 0.7699 0.7708
> > 64KB 512MB 1M 0.7799 0.7694
> >
> > This data appears to show some value
> > in capping for 1500b lengths and noise
> > for shorter and longer lengths.
> >
> > Any idea what the actual distribution of
> > do_csum lengths is under various loads?
> >
> I don't have any hard data no, sorry.

I think you should before you implement this.
You might find extremely short lengths.

> I'll cap the prefetch at 1500B for now, since it
> doesn't seem to hurt or help beyond that

The table data has a max prefetch of
8 * boot_cpu_data.x86_cache_alignment so
I believe it's always less than 1500 but
perhaps 4 might be slightly better still.




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