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Re: [PATCH] Change PRED_LOOP_EXIT from 85 to 92
On 12/24/2017 05:03 AM, Segher Boessenkool wrote:
> On Sun, Dec 24, 2017 at 09:12:56AM +0000, Richard Sandiford wrote:
>> Segher Boessenkool <segher@kernel.crashing.org> writes:
>>> On Fri, Dec 22, 2017 at 04:53:47PM -0600, David Esparza wrote:
>>>> With a value of 85 GCC has a CPU performance degradation of 11%,
>>>> reverting PRED_LOOP_EXIT to 92 this degradation disappear.
>>>> Those values where measured by running c-ray ray-tracer that is a
>>>> floating point benchmark that runs out of L1 cache.
>>>
>>> Why is this single benchmark more important than everything else?
>>>
>>> https://patchwork.ozlabs.org/patch/637073/
>>
>> "Everything" else? :-) It sounds from Andrew's reply like it wasn't
>> a win on other benchmarks too.
>
> Yeah... But at least Martin tested spec2006, instead of one single
> tiny non-representative program.
>
>> Neither covering message has really explained why the previous value was
>> too low/high, but maybe that's just the way it goes with these tuning
>> parameters...
>
> It would be nice if they explained how they tested things.
Agreed. I can't see any way for this patch to go forward without some
explanation of why it helps this particular c-ray implementation and
some data showing it's not hurtful on a wider suite of benchmarks.
Jeff