Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

[Naveen N. Rao]

Hi Nathan,

Nathan Lynch wrote:
> Hi Kamalesh,
>
> Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx> writes:
> > On 12/5/19 3:54 AM, Nathan Lynch wrote:
> > > "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx> 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