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SubjectRe: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks
Hi Nathan,

Nathan Lynch wrote:
> Hi Kamalesh,
>
> Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> writes:
>> On 12/5/19 3:54 AM, Nathan Lynch wrote:
>>> "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> writes:
>>>>
>>>> Tools such as lparstat which are used to compute the utilization need
>>>> to know [S]PURR ticks when the cpu was busy or idle. The [S]PURR
>>>> counters are already exposed through sysfs. We already account for
>>>> PURR ticks when we go to idle so that we can update the VPA area. This
>>>> patchset extends support to account for SPURR ticks when idle, and
>>>> expose both via per-cpu sysfs files.
>>>
>>> Does anything really want to use PURR instead of SPURR? Seems like we
>>> should expose only SPURR idle values if possible.
>>>
>>
>> lparstat is one of the consumers of PURR idle metric
>> (https://groups.google.com/forum/#!topic/powerpc-utils-devel/fYRo69xO9r4).
>> Agree, on the argument that system utilization metrics based on SPURR
>> accounting is accurate in comparison to PURR, which isn't proportional to
>> CPU frequency. PURR has been traditionally used to understand the system
>> utilization, whereas SPURR is used for understanding how much capacity is
>> left/exceeding in the system based on the current power saving mode.
>
> I'll phrase my question differently: does SPURR complement or supercede
> PURR? You seem to be saying they serve different purposes. If PURR is
> actually useful rather then vestigial then I have no objection to
> exposing idle_purr.

SPURR complements PURR, so we need both. SPURR/PURR ratio helps provide
an indication of the available headroom in terms of core resources, at
maximum frequency.


- Naveen

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