Re: perf pebs sampling through stores + period is wrong?

[Harald Servat <harald.servat@bsc.es>]

On 20/02/14 22:27, Andi Kleen wrote:
> Harald Servat <harald.servat@bsc.es> writes:
>>
>>    $ perf mem -t store record -c 10000 ./a.out
>>    ...
>>    [perf record: Woken up 4 times to write data]
>>    [perf record: Captured and wrote 0.921 MB perf.data (~40247 samples)]
>>
>>    Notice that the number of samples raised by 20x, which to me seems
>> very odd because the number of stores was half, so I expected 0.5x
>> here. Or am I supposing this the wrong way?
>
> Likely you're throttling. 10k is a far too low period for such
> measurements
>
> (The cpu can do multiple stores per cycle and it runs at multiple
> Ghz. Each PMI takes many thousands of cycles. You can do the math.)
>
> -Andi
>

Dear Andi,

   but then why the loads aren't throttling? There are far more loads in 
the app than stores (as seen in the perf stat results), but the loads do 
not throttle while store do? Of course, apps face different performance 
rate as apps run and there may be situations where the number of 
loads/second is either larger or smaller than stores/second, but still 
it is a bit confusing why so much difference between loads & stores. I 
would expect also the loads to throttle, then.

Regards.


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