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

[Jesse Barnes]

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.

> So we want to race-to-idle, except for when we don't want to
> race-to-idle. Seems like a good reason to keep improving our measuring
> tools.

Always.

Jesse