When I call GetBufferSubData with my Shader Storage Buffer Object there is typically a 4ms delay. Is it possible for my application to do work during that time?
// start GetBufferSubData
// do client/app/CPU work
// (wait if needed)
// read results from GetBufferSubData
Or otherwise use some sort of API to asynchronously start copying buffer data from the GPU?
I was able to get an async readback working using glMapBufferRange and GL_MAP_PERSISTENT_BIT. However, when running a compute shader (multiple times back to back) on that buffer, this results in a massive performance degradation compared to no persistent mapping.
The issue with simply marking the buffer with GL_MAP_PERSISTENT_BIT was that this resulted in a substantial performance degradation (8x slower) when running a compute shader on that buffer (profiled using Nvidia Nsight Graphics). I suspect this is because of the mapping, OpenGL needs to read/write the buffer into a different location that is less performant on the GPU, but more performant/accessible by the CPU.
My solution was to create a much smaller buffer (1000x smaller, 16kb) that is persistently mapped that the CPU can use to read/write to the larger buffer in small increments when needed. This combination was much faster with only a minor API overhead that achieved my needs.