dabbill@Archlinux ~ $ glxgears
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
303 frames in 5.0 seconds = 60.485 FPS
301 frames in 5.0 seconds = 60.173 FPS
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0.0"
      after 1959 requests (1959 known processed) with 0 events remaining.
dabbill@Archlinux ~ :( $ vblank_mode=0 glxgears
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: option value of option vblank_mode ignored.
11915 frames in 5.0 seconds = 2382.896 FPS
11478 frames in 5.0 seconds = 2295.576 FPS
12041 frames in 5.0 seconds = 2408.154 FPS
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0.0"
      after 76507 requests (76507 known processed) with 0 events remaining.
dabbill@Archlinux ~ :( $ vblank_mode=0 primus glxgears
bash: primus: command not found
dabbill@Archlinux ~ :( $ vblank_mode=0 primusrun glxgears
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: option value of option vblank_mode ignored.
9834 frames in 5.0 seconds = 1966.650 FPS
10074 frames in 5.0 seconds = 2014.654 FPS
9968 frames in 5.0 seconds = 1993.503 FPS
8088 frames in 5.0 seconds = 1617.384 FPS
8702 frames in 5.0 seconds = 1740.293 FPS
^C
dabbill@Archlinux ~ :( $ vblank_mode=0 optirun glxgears
4426 frames in 5.0 seconds = 884.998 FPS
4473 frames in 5.0 seconds = 894.578 FPS
4387 frames in 5.0 seconds = 877.373 FPS
[VGL] ERROR: in readback--
[VGL]    246: Window has been deleted by window manager
dabbill@Archlinux ~ :( $