[Intel-gfx] [PATCH 2/3] drm/i915: Boost RPS frequency for CPU stalls

Chris Wilson chris at chris-wilson.co.uk
Thu Oct 3 01:19:14 CEST 2013

On Wed, Oct 02, 2013 at 03:20:23PM -0700, Jesse Barnes wrote:
> On Wed, 2 Oct 2013 15:18:20 -0700
> Jesse Barnes <jbarnes at virtuousgeek.org> wrote:
> > On Wed, 2 Oct 2013 22:57:41 +0100
> > Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > No, that is a factor I had not considered. I had been concerned about
> > > monitoring latency whilst minimising frequency and using the power gauge
> > > as the ultimate measure of success. One thing that is apparent with ivb,
> > > is that the power gauge (at least) is dependent upon workload. That is
> > > you can set high frequencies, but if the ring is idle (but not in rc6)
> > > then it consumes relatively little power. (That still may be 1W on the
> > > big GPUs, but it does mean that at low frequencies there is not a lot of
> > > difference between rc6 on/off, a few 10s of mW, and we are concerned
> > > with a window of about 10ms.) I suspect the difference is likely to be
> > > in the noise for an idle desktop workload and would only really show
> > > itself in a synthetic benchmark.
> > 
> > Hm are you sure about that?  It doesn't match the last measurements I
> > had from a long time ago... until RC6 kicked in the GPU burned quite a
> > bit (several Watts).
> > 
> > If you're correct though, then I agree, this is quibbling over crumbs,
> > so not worth modifying.
> Oh and the other thing that comes to mind is platform power.  Even if
> RC6 vs no RC6 at low freq is minimally different, entering RC6 allows
> the platform to go into a deep sleep state if the CPUs are also idle.
> So race to idle as efficiently as possible is probably still the right
> answer.

Right, I'm basing my judgement purely on the energy msr, I have no idea
how that actually relates to instantaneous power consumption from the
battery/mains. I only hope the correlation is good (because that msr is
so convenient to measure and record in realtime).

I've been deferring to Mengmeng for system power measurements - its
about time I actually started measuring those for myself.

Chris Wilson, Intel Open Source Technology Centre

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