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SubjectRe: [PATCH V3 2/2] powerpc/pseries: init fault_around_order for pseries
On Wednesday 30 April 2014 12:34 PM, Rusty Russell wrote:
> Ingo Molnar <mingo@kernel.org> writes:
>> * Madhavan Srinivasan <maddy@linux.vnet.ibm.com> wrote:
>>
>>> Performance data for different FAULT_AROUND_ORDER values from 4 socket
>>> Power7 system (128 Threads and 128GB memory). perf stat with repeat of 5
>>> is used to get the stddev values. Test ran in v3.14 kernel (Baseline) and
>>> v3.15-rc1 for different fault around order values.
>>>
>>> FAULT_AROUND_ORDER Baseline 1 3 4 5 8
>>>
>>> Linux build (make -j64)
>>> minor-faults 47,437,359 35,279,286 25,425,347 23,461,275 22,002,189 21,435,836
>>> times in seconds 347.302528420 344.061588460 340.974022391 348.193508116 348.673900158 350.986543618
>>> stddev for time ( +- 1.50% ) ( +- 0.73% ) ( +- 1.13% ) ( +- 1.01% ) ( +- 1.89% ) ( +- 1.55% )
>>> %chg time to baseline -0.9% -1.8% 0.2% 0.39% 1.06%
>>
>> Probably too noisy.
>
> A little, but 3 still looks like the winner.
>
>>> Linux rebuild (make -j64)
>>> minor-faults 941,552 718,319 486,625 440,124 410,510 397,416
>>> times in seconds 30.569834718 31.219637539 31.319370649 31.434285472 31.972367174 31.443043580
>>> stddev for time ( +- 1.07% ) ( +- 0.13% ) ( +- 0.43% ) ( +- 0.18% ) ( +- 0.95% ) ( +- 0.58% )
>>> %chg time to baseline 2.1% 2.4% 2.8% 4.58% 2.85%
>>
>> Here it looks like a speedup. Optimal value: 5+.
>
> No, lower time is better. Baseline (no faultaround) wins.
>
>
> etc.
>
> It's not a huge surprise that a 64k page arch wants a smaller value than
> a 4k system. But I agree: I don't see much upside for FAO > 0, but I do
> see downside.
>
> Most extreme results:
> Order 1: 2% loss on recompile. 10% win 4% loss on seq. 9% loss random.
> Order 3: 2% loss on recompile. 6% win 5% loss on seq. 14% loss on random.
> Order 4: 2.8% loss on recompile. 10% win 7% loss on seq. 9% loss on random.
>
>> I'm starting to suspect that maybe workloads ought to be given a
>> choice in this matter, via madvise() or such.
>
> I really don't think they'll be able to use it; it'll change far too
> much with machine and kernel updates. I think we should apply patch #1
> (with fixes) to make it a variable, then set it to 0 for PPC.
>

Ok. Will do.

Thanks for review
With regards
Maddy


> Cheers,
> Rusty.
>



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