Android GPU profiling - OpenGL Live Wallpaper is s

Question

I'm developing a Live Wallpaper using OpenGL ES 3.0. I've set up according to the excellent tutorial at http://www.learnopengles.com/how-to-use-opengl-es-2-in-an-android-live-wallpaper/, adapting GLSurfaceView and using it inside the Live Wallpaper.

I have a decent knowledge of OpenGL/GLSL best practices, and I've set up a simple rendering pipeline where the draw loop is as tight as possible. No re-allocations, using one static VBO for non-changing data, a dynamic VBO for updates, using only one draw call, no branching in the shaders et cetera. I usually get very good performance, but at seemingly random but reoccurring times, the framerate drops.

Profiling with the on-screen bars gives me intervals where the yellow bar ("waiting for commands to complete") shoots away and takes everything above the critical 60fps threshold.

I've read any resources on profiling and interpreting those numbers I can get my hands on, including the nice in-depth SO question here. However, the main takeaway from that question seems to be that the yellow bar indicates time spent on waiting for blocking operations to complete, and for frame dependencies. I don't believe I have any of those, I just draw everything at every frame. No reading.

My question is broad - but I'd like to know what things can cause this type of framerate drop, and how to move forward in pinning down the issue.

Here are some details that may or may not have impact:

• I'm rendering on demand, onOffsetsChanged is the trigger (render when dirty).
• There is one single texture (created and bound only once), 1024x1024 RGBA. Replacing the one texture2D call with a plain vec4 seems to help remove some of the framerate drops. Reducing the texture size to 512x512 does nothing for performance.
• The shaders are not complex, and as stated before, contain no branching.
• There is not much data in the scene. There are only ~300 vertices and the one texture.
• A systrace shows no suspicious methods - the GL related methods such as buffer population and state calls are not on top of the list.

Update: As an experiment, I tried to render only every other frame, not requesting a render every onOffsetsChanged (swipe left/right). This was horrible for the look and feel, but got rid of the yellow lag spikes almost completely. This seems to tell me that doing 60 requests per frame is too much, but I can't figure out why.

Answer 1

My question is broad - but I'd like to know what things can cause this type of framerate drop, and how to move forward in pinning down the issue.

(1) Accumulation of render state. Make sure you "glClear" the color/depth/stencil buffers before you start each render pass (although if you are rendering directly to the window surface this is unlikely to be the problem, as state is guaranteed to be cleared every frame unless you set EGL_BUFFER_PRESERVE).

(2) Buffer/texture ghosting. Rendering is deeply pipelined, but OpenGL ES tries to present a synchronous programming abstraction. If you try to write to a buffer (SubBuffer update, SubTexture update, MapBuffer, etc) which is still "pending" use in a GPU operation still queued in the pipeline then you either have to block and wait, or you force a copy of that resource to be created. This copy process can be "really expensive" for large resources.

(3) Device DVFS (dynamic frequency and voltage scaling) can be quite sensitive on some devices, especially for content which happens to sit just around a level decision point between two frequencies. If the GPU or CPU frequency drops then you may well get a spike in the amount of time a frame takes to process. For debug purposes some devices provide a means to fix frequency via sysfs - although there is no standard mechnanism.

(4) Thermal limitations - most modern mobile devices can produce more heat than they can dissipate if everything is running at high frequency, so the maximum performance point cannot be sustained. If your content is particularly heavy then you may find that thermal management kicks in after a "while" (1-10 minutes depending on device, in my experience) and forcefully drops the frequency until thermal levels drop within safe margins. This shows up as somewhat random increases in frame processing time, and is normally unpredictable once a device hits the "warm" state.

If it is possible to share an API sequence which reproduces the issue it would be easier to provide more targeted advice - the question is really rather general and OpenGL ES is a very wide API ;)