I am here to report a potential bug:
It seems like NVIDIA’s driver can commit up to several megabytes of RAM (CPU ram, not VRAM) for linked GLSL shader programs. As one would suspect, the amount of consumed RAM is somewhat proportional to the complexity of the shader program. However, it still seems much higher than it needs to be. For example, some of our more complex shader programs easily exceed 2MB. When dealing with high quantity of shaders, this becomes a huge problem.
In our application we generate shaders dynamically and they often end up being quite complex (Example Vertex and Fragment shader). Furthermore, we deal with large amounts of shaders, in the range of 5k to 20k. The problem we are facing is that the graphics driver allocates up to 15GB of RAM just for compiled shaders. The question is, is this intended behavior or a bug? We already double and triple checked to make sure this is not a mistake on our end.
I wrote a test application to demonstrate the issue. Source is available here (VS2015). It links one set of vertex + fragment shader 1000 times and then prints the amount of RAM commited by the application. The application itself does not allocate any extra memory. Additionally, the .zip comes with multiple sets of example shaders taken from our application to see the difference in RAM usage. For more details see main.cpp
Some other observations I made:
- Occurs on all driver versions and all Windows versions
- RAM usage is proportional to complexity of shader (no surprise here)
- Conditionals (if clauses and '?' operator) seem to massively increase RAM usage and compile times
- The size of uniform buffer arrays only slightly affect RAM usage
- Detaching and deleting shaders (glDetachShader+glDeleteShader) after glLinkProgram helps only a bit
- Calling glDeleteProgram() correctly releases all memory, indicating there is no leak
- Same problem occurs when the shader programs are loaded via glProgramBinary
Thanks in advance!
Edit: We ran the test application on a different set of GPUs from all three vendors (Nvidia, AMD and Intel). Results are here.