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SubjectRe: [PATCH v2 2/2] x86: add prefetching to do_csum
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:

With limiting:
1500B | 64MB | 1000000 | 1.344167
1500B | 128MB | 1000000 | 1.340970
1500B | 256MB | 1000000 | 1.353562
1500B | 512MB | 1000000 | 1.346349
9000B | 64MB | 1000000 | 0.852174
9000B | 128MB | 1000000 | 0.852765
9000B | 256MB | 1000000 | 0.853153
9000B | 512MB | 1000000 | 0.852661
64KB | 64MB | 1000000 | 0.768585
64KB | 128MB | 1000000 | 0.769465
64KB | 256MB | 1000000 | 0.769909
64KB | 512MB | 1000000 | 0.779895


Without limiting

1500B | 64MB | 1000000 | 1.360525
1500B | 128MB | 1000000 | 1.354220
1500B | 256MB | 1000000 | 1.371037
1500B | 512MB | 1000000 | 1.353557
9000B | 64MB | 1000000 | 0.850415
9000B | 128MB | 1000000 | 0.853642
9000B | 256MB | 1000000 | 0.852048
9000B | 512MB | 1000000 | 0.852484
64KB | 64MB | 1000000 | 0.768261
64KB | 128MB | 1000000 | 0.768566
64KB | 256MB | 1000000 | 0.770822
64KB | 512MB | 1000000 | 0.769391

Doesn't look like much consistent improvement.

Neil



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