Re: [PATCH kvm-unit-tests] pmu: fixes for Sandy Bridge hosts

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/06/2013 08:38, Gleb Natapov ha scritto:
> On Mon, Jun 03, 2013 at 08:33:13AM +0200, Paolo Bonzini wrote:
>> Il 02/06/2013 17:32, Gleb Natapov ha scritto:
>>> On Thu, May 30, 2013 at 07:43:07PM +0200, Paolo Bonzini wrote:
>>>> This patch includes two fixes for SB:
>>>>
>>>> * the 3rd fixed counter ("ref cpu cycles") can sometimes report
>>>>   less than the number of iterations
>>>>
>>> Is it documented? It is strange for "architectural" counter to behave
>>> differently on different architectures.
>>
>> It just counts the CPU cycles.  If the CPU can optimize the loop better,
>> it will take less CPU cycles to execute it.
>>
> We should try and change the loop so that it will not be so easily optimized.
> Making the test succeed if only 10% percent of cycles were spend on a loop
> may result in the test missing the case when counter counts something
> different.

Any hard-to-optimize loop risks becoming wrong on the other side (e.g.
if something stalls the pipeline, a newer chip with longer pipeline will
use more CPU cycles).

Turbo boost could also contribute to lowering the number of cycles; a
boosted processor has ref cpu cycles that are _longer_ than the regular
cycles (thus they count in smaller numbers).  Maybe that's why "core
cycles" didn't go below N.

The real result was something like 0.8*N (780-830000).  I used 0.1*N
because it is used for the "ref cpu cycles" gp counter, which is not the
same but similar.  Should I change it to 0.5*N or so?

Paolo