[Intel-gfx] Strange performance cliff...

[Peter Clifton]

On Tue, 2010-10-12 at 18:45 +0100, Peter Clifton wrote:
> Using glxgears as a tool to exercise the GPU with some simple rendering,
> I have noted a strange cliff in the intel_gpu_top output when resizing
> the glxgears window:
> 
> Below a certain size e.g.:
> 
>   -geometry 576x868+0+29

>                    render busy:  21%: ████▎                                  render space: 10/126976 (0%)
> When I increase the window size just a fraction, to:
> 
>   -geometry 576x871+0+29

> The CS (command streamer) unit jumps to 100% busy, along with the render
> busy graph. Does anyone have any ideas why?
> 
>                    render busy: 100%: ████████████████████                   render space: 61/126976 (0%)


It would appear that this transition occurs at the point when glxgears
executed with vblank_mode=0 set, gives approx 60Hz (vblank rate).

I can only presume therefore, that the command stream shows as 100% when
it is stalling the CPU.. is that right?

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)
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