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

* Neil Horman <nhorman@tuxdriver.com> wrote:

> Base:
> 0.093269042 seconds time elapsed ( +- 2.24% )
> Prefetch (5x64):
> 0.079440009 seconds time elapsed ( +- 2.29% )
> Parallel ALU:
> 0.087666677 seconds time elapsed ( +- 4.01% )
> Prefetch + Parallel ALU:
> 0.080758702 seconds time elapsed ( +- 2.34% )
>
> So we can see here that we get about a 1% speedup between the base
> and the both (Prefetch + Parallel ALU) case, with prefetch
> accounting for most of that speedup.

Hm, there's still something strange about these results. So the
range of the results is 790-930 nsecs. The noise of the measurements
is 2%-4%, i.e. 20-40 nsecs.

The prefetch-only result itself is the fastest of all -
statistically equivalent to the prefetch+parallel-ALU result, within
the noise range.

So if prefetch is enabled, turning on parallel-ALU has no measurable
effect - which is counter-intuitive. Do you have an
theory/explanation for that?

Thanks,

Ingo


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