[Intel-gfx] FPS performance increase when deliberately spinning the CPU with an unrelated task

[Jesse Barnes]

On Sat, 23 Oct 2010 13:02:35 +0100
Peter Clifton <pcjc2 at cam.ac.uk> wrote:
> I think Keith was thinking that there are some parts of the chipset
> which are shared between the GPU and CPU (memory controllers?), and the
> CPU entering a lower frequency state could have a detrimental effect on
> the graphics throughput.
> 
> I know in heavy workloads the CPU is likely to be "a bit" busy, and
> rendering will not be totally GPU bound, but it would seem like it is
> eventually necessary to have some hook to bump the CPU frequency (or
> chipset frequency?) when the GPU would make beneficial use of the extra
> throughput.
> 
> This doesn't make sense if it is banging out 100fps, but for my stuff,
> the GPU is struggling to make 5fps for some complex circuit boards. I'm
> trying to address that from a geometry / rendering complexity point of
> view, but also, I'd love to see my laptop being able to get the best out
> of its hardware.
> 
> Perhaps we need to account for periods when the CPU has tasks idle
> waiting for GPU operations which would be sped up by increasing some
> chip power state.
> 
> I'm probably not up to coding this all, but if the idea sounds feasible,
> I'd love to know, so I might be able to have a tinker with it.

There are some bits in the GMCH to control memory behavior during CPU
C-states.  Can you dump the 16 bits at MCHBAR address 0xf08?  You
should be able to do that by doing I915_READ16(MCHBAR_MIRROR_BASE +
0xf08).  Assuming bits 3:2 and 1:0 are nonzero, it may help to set them
all to 0.  That will disable several memory related power saving
features while the CPU is in a deep sleep state.

-- 
Jesse Barnes, Intel Open Source Technology Center