I've got a multithreaded OpenGL application using PBOs for data transfers between cpu and gpu.
I have pooled the allocation of PBOs, however, when the pools are empty my non-opengl threads have to block for a while until the OpenGL thread reaches a point where it can allocate the buffers (i.e. finished rendering the current frame). This waiting is causing some lag spikes in certain situations which I'd like to avoid.
Is it possible to allocate PBO's on another thread which are then used by the "main" OpenGL thread?
Yes, you can create objects on one thread which can be used on another. You'll need a new GL context to do this.
That being said, there are two concerns.
First, it depends on what you mean by "allocation of PBOs". You should never be allocating buffer objects in the middle of a frame. You should allocate all the buffers you need up front. When the time comes to use them, then you can simply use what you have.
By "allocate", I mean call glBufferData on a previously allocated buffer using a different size or driver hint than was used before. Or by using glGenBuffers and glDeleteBuffers in any way. Neither of these should happen within a frame.
Second, invalidating the buffer should never cause "lag spikes". By "invalidate", I mean reallocating the buffer with glBufferData using the same size and usage hint, or using glMapBufferRange with the GL_INVALIDATE_BUFFER bit. You should look at this page on how to stream buffer object data for details. If you're getting issues, then you're probably on NVIDIA hardware and using the wrong buffer object hint (ie: use STREAM).