[phil@f15 ~]$ vblank_mode=0 glxgears
ATTENTION: default value of option vblank_mode overridden by environment.
ATTENTION: default value of option vblank_mode overridden by environment.
314 frames in 5.0 seconds = 62.729 FPS
386 frames in 5.0 seconds = 77.187 FPS
396 frames in 5.0 seconds = 79.022 FPS
449 frames in 5.0 seconds = 89.776 FPS
393 frames in 5.0 seconds = 78.580 FPS
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 2312 requests (2312 known processed) with 0 events remaining.
[phil@f15 ~]$ glxgears
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 34 requests (34 known processed) with 0 events remaining.