How to interpret Profile Results

[Rafael Spring]

Hello,

we are currently doing some performance testing of our GPU code and have
found that apitrace (a tool we really appreciate!) contains a profiling
tool that works with OpenGL.

However we are a bit unsure how to interpret the results. According to the
Profile Results a lot of GPU time seems to be spent in draw calls of a few
simple shaders that don't draw a lot of pixels, whereas the complex shaders
that draw lots of pixels seem to be fairly quick. Also some of the GPU draw
call times seem to vary by orders of magnitude for the same shader and
operation (i.e. one call takes ~20 us and the next draw call, performing
the exact same operation with the same shader but possibly to a different
part of the framebuffer, takes 10000 us).
We suspect the execution times aren't always true compute times but may
contain stall times. Another theory is that some of the draw call times are
actually just scheduling times and the real computation time ends up
counting towards another draw call timing, when a batch of operations gets
executed.

Any insight into how to interpret the Profile Results and how to derive
optimization insight from it would be greatly appreciated.

For testing we are currently using a MS Surface Pro 3 tablet running
Windows 8.1.

Thanks,
Rafael


-- 
Rafael Spring
CTO, Co-Founder
DotProduct LLC / DotProduct GmbH
rafael at dotproduct3d.com
www.dotproduct3d.com
m: +49 1525 3822738
@DotProduct3D
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