I did some experiments with vsync:
VSYNC ON: 60 fps, 6% of 1 CPU core.
VSYNC OFF: 150 fps, 100% of 1 CPU core.
I profiled the run, and with vsync off, I see glXSwapBuffers() take 96% of the CPU cycles.
The FPS delta (150/60) is just a factor 2.5 so it would be doing, at most, 2.5 times the work on the CPU. But the CPU usage goes from nothing to 100%.
What is going on here?
OpenGL: 3.2 Core Profile
Driver: 4.6.0 NVIDIA 455.23.05
Why does the buffer swap go from no-cost to a 100% CPU cost?