When running a flowgraph that includes both a scopesink and an fftsink, I get the following warning on the terminal:

do_wait: drmWaitVBlank returned -1, IRQs don't seem to be working correctly.
Try adjusting the vblank_mode configuration parameter.

Disabling Compiz makes no difference. Disabling any one of the two graphical sinks makes the warning go away, so it seems it’s the combination of the two that provokes it. I’m on an Intel 945GM graphics chip.

This vblank_mode parameter is found in ~/.drirc (doesn’t exist normally). You can use driconf to create it and modify the parameters.

None of the possible values for vblank_mode [0, 1, 2, 3] made the warning go away.