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

[Naveen N. Rao]

Naveen N. Rao wrote:
> 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.

Re-reading this today morning, I realize that this isn't entirely 
accurate. SPURR alone is sufficient to understand core resource 
utilization.

Kamalesh is using PURR to display non-normalized utilization values 
(under 'actual' column), as reported by lparstat on AIX. I am not 
entirely sure if it is ok to derive these based on the SPURR busy/idle 
ratio.

- Naveen