linux-kernel - Re: Use case for PERF_COUNT_HW_REF_CPU

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Date:	Wed, 8 Feb 2012 12:08:08 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu
Subject: Re: Use case for PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event

On Wed, Feb 8, 2012 at 11:51 AM, Anshuman Khandual
<khandual@...ux.vnet.ibm.com> wrote:
> On Wednesday 08 February 2012 03:21 PM, Stephane Eranian wrote:
>> On Wed, Feb 8, 2012 at 10:43 AM, Anshuman Khandual
>> <khandual@...ux.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.
>
I was just listing the what you could do with the event. I have not
yet tried to measure
idle with ref-cycles.

>
>>
>> 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
>>>
>>
>
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