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Re: [PATCH] Change PRED_LOOP_EXIT from 85 to 92


On 01/02/2018 04:57 AM, Martin Liška wrote:
> On 12/24/2017 07:58 PM, Jeff Law wrote:
>> 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
>>
> 
> Hello.
> 
> Sorry for late answer. I've been currently working on returning of predictors
> according to SPEC2006 and SPE2017. I've got prepared patches and currently testing
> it's impact on speed of SPEC benchmarks.
> 
> From what I've measured, suggested adjustment for this particular predictor would be
> increase to 89.
> 
> Note that this predictor has quite high coverage on SPEC: 5.3% (of all branching).
Understood.  I think using SPEC to guide here makes much more sense.

Jeff


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