OpenGL code slow on one computer (but not on others) [closed]

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Closed 8 years ago.

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I have a shader that is currently doing some raytracing. The shader used to take the scene information as uniforms to render the scene but this proved to be way too limited so we switched to using SSBOs (shader storage buffer objects). The code works perfectly on two computers, but another computer is rendering it very slowly. The video card for that computer is a radeon HD 6950. The video cards that are rendering it correctly are a GTX 570 and a radeon HD 7970. The scene is shown correctly on the three computers but the radeon HD 6950 is rendering it very slowly (1 FPS when we are rotating around the scene). We thought it was a openGL version problem but it doesn't seem to be the case since we updated the drivers and it still doesn't work. Any idea where the problem might be?

Answer 1

There are a few possibilities:

1. You could be falling off the fast path on that particular card. Some aspect of your rendering may not be implemented as efficiently on the lower-end card, for example.
2. You may be hitting the VRAM limit on the 6950, but not on the other 2 cards and OpenGL is essentially thrashing, swapping things out to main memory and back
3. You may have triggered software rendering on that card. There may be some specific OpenGL feature you're using that's only implemented in software for the 6950, but is hardware accelerated on the other cards.

You don't say which OS you're working with, so I'm not sure what to tell you about debugging the problem. On MacOS you can use OpenGL Profiler to see if it's falling back to software and use OpenGL Driver Monitor to see if it's paging out. On iOS you can use Xcode's OpenGL profiling instrument for both of those. I'm not sure on Windows or Linux as I don't have experience with them.