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What is the GPU/driver doing if not shading?

Boian Petkantchin

I ran Streamline on the SimpleTriangle example from Mali OpenGL ES SDK for Android v1.6.0. Basically, each frame it renders a triangle, that covers half of the frame. It renders to the default framebuffer. What I observe is, that most of the time is spent not vertex/fragment processing. What is actually the GPU/driver doing during this time? Note that I don't mean the time between frames, but the time between vertex and fragment processing.

I have tried this example on two platforms, one with Mali-400 and the other with Mali-450. Both give the same result.

Below is an illustration of the behavior when rendering a single frame. As you can see, the middle part is a significant portion of the processing of the frame.

what-is-the-gpu-doing-part.png

Below is a trace of the OpenGL API calls for a single frame.

glClearColor(red=0.0, green=0.0, blue=0.0, alpha=1.0)

glClear(mask=GL_DEPTH_BUFFER_BIT|GL_COLOR_BUFFER_BIT)

glUseProgram(program=3)

glVertexAttribPointer(index=0, size=2, type=GL_FLOAT, normalized=GL_FALSE, stride=0, pointer=0x776e3af0)

glEnableVertexAttribArray(index=0)

glDrawArrays(mode=GL_TRIANGLES, first=0, count=3)

eglSwapBuffers(dpy=0x1, surface=0x77474988)

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  • 0 in reply to Michael McGeagh

    This seams reasonable, one thing though. Data is sampled with 1KHz frequency. Doesn't this mean that the Fragment Processor counters should measure an increase at least once during this 2.8 ms window? I never observe this. Never are the counters increased in the beginning of fragment activity, always at the end, so it can't be a matter of occasional drop in sampling frequency.

    As you can see in the image, vertex activity is aways between Vertex Processor counters increases. This is what I expect to see for the Fragment activity as well.

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  • 0 in reply to Michael McGeagh

    This seams reasonable, one thing though. Data is sampled with 1KHz frequency. Doesn't this mean that the Fragment Processor counters should measure an increase at least once during this 2.8 ms window? I never observe this. Never are the counters increased in the beginning of fragment activity, always at the end, so it can't be a matter of occasional drop in sampling frequency.

    As you can see in the image, vertex activity is aways between Vertex Processor counters increases. This is what I expect to see for the Fragment activity as well.

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