~$ 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 503 requests (503 known processed) with 0 events remaining.

~$ glxdemo
Redraw event
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 40 requests (40 known processed) with 0 events remaining.


    ~$glxheads
glxheads: exercise multiple GLX connections (any key = exit)
Usage:
  glxheads xdisplayname ...
Example:
  glxheads :0 mars:0 venus:1
Name: :0
  Display:     0x19e3420
  Window:      0x3800002
  Context:     0x19fafc0
  GL_VERSION:  3.0 Mesa 11.2.0
  GL_VENDOR:   nouveau
  GL_RENDERER: Gallium 0.4 on NVCF
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 1358 requests (1358 known processed) with 0 events remaining.