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SubjectRe: [PATCH] x86: Run checksumming in parallel accross multiple alu's
On Mon, Oct 28, 2013 at 05:20:45PM +0100, Ingo Molnar wrote:
>
> * 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,
I mentioned it farther down, loosely theorizing that running with parallel
alu's in conjunction with a prefetch, puts more pressure on the load/store unit
causing stalls while both alu's wait for the L1 cache to fill. Not sure if that
makes sense, but I did note that in the both (prefetch+alu case) our data cache
hit rate was somewhat degraded, so I was going to play with the prefetch stride
to see if that fixed the situation. Regardless I agree, the lack of improvement
in the both case is definately counter-intuitive.

Neil

>
> Ingo
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