glReadPixels into PBO is blocking?

I have to read back framebuffer data (both color and depth) every frame. I  
don't care about latency, but I don't want to block the CPU at all. For  
this reason, I've used a ring of PBOs, where I would read into a new PBO  
every frame, and only touch it after a couple frames later, to avoid  
blocking. (As discussed in this paper:  
http://developer.nvidia.com/object/fast_texture_transfers.html)

Much to my surprise, this doesn't seem to work efficiently. glReadPixels  
seems to use up all the CPU, even when reading into a PBO. I have created  
a bare bones test application: http://www.andrasbalogh.com/gltest.zip to  
demonstrate it (includes executable, source and project files). The test  
application renders a single triangle on screen, then reads back a 1x1  
pixel area. In this sample application, I don't use the PBO ring, to keep  
it simple, but I never actually read the data from the PBO. I just read  
into it, so it should be a 100% asynchronous operation. In the worst case  
it should decrease framerate, but should not use this much CPU.

Commenting out the glReadPixels call, my CPU usage drops to 1%. When I put  
glFinish() right before calling glReadPixels(), then all the time is spent  
in glFinish(), and glReadPixels is basically free (I've measured this  
using RDTSC on a single core computer). This makes me suspect that there's  
an implicit glFinish() in ReadPixels(), which makes sense, when reading  
into client memory, but does not, when reading into a PBO.

I've also noticed similar behaviour, when I use glCopyTexSubImage2D() to  
copy into a texture from a FBO.

The thing that puzzles me the most, is that if I actually take the  
glReadPixels() out, and just replace it with a glFinish() call, it still  
makes my CPU run at a 100%! How is that possible? Even if glFinish() is  
polling, it should return way before the vsync, and during SwapBuffers()  
my CPU should be idle. Or am I missing something?

I've tested multiple usage types (DYNAMIC_READ, STREAM_READ, STATIC_READ),  
multiple formats, multiple region sizes, and there's no difference.
I've tested on a GeForce6600 and a GeForce7600GS on WinXP with drivers  
93.71 and 93.81.

Any ideas what I'm doing wrong?

Thanks,


- Andras
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Author: Andras Balogh
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