Re: Use case for PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On Wednesday 08 February 2012 03:21 PM, Stephane Eranian wrote:
> On Wed, Feb 8, 2012 at 10:43 AM, Anshuman Khandual
> <khandual@linux.vnet.ibm.com> wrote:
>> Hello Stephane,
>>
>> I was going through the following discussion where we added the
>> new HW generic event PERF_COUNT_HW_REF_CPU_CYCLES.
>>
>> https://lkml.org/lkml/2011/12/10/103
>>
>> (Sorry, for asking this question bit late)
>>
>> I am trying to understand the use case for this. Would this new event
>> help us in generating (during a perf session) a CPU frequency invariant
>> time metric against which we would plot our other perf event's measurements ?
>> CPU frequency independent time measurement is it's primary purpose ? or we were
>> finding a way to expose the fixed counter 2 which was not getting used before
>> for not having an event encoding. I guess this would help us in finding equivalent
>> PMU events or mechanisms in other architecture / platforms.
>>
> The goal was to expose a cycle event that is not subject to frequency scaling
> nor turbo boost of any sort. An event that could be used to correlate with time.

> An event that could also be used to compute idle time by comparing its value
> with wall-clock time.

Why kernel computed idle time is not sufficient ? How much accuracy would it
improve in using PMU event computed idle time over kernel computed idle time. 


> 
> The fact that on Intel X86 this event is on fixed counter 2 is an
> implementation
> detail.
> 
>> --
>> Anshuman Khandual
>> Linux Technology Centre
>> IBM Systems and Technology Group
>>
>