GtkGlArea render fps is different than monitor frame rate

[Ferenc Engárd <ferenc engard gmail com>]

Hi,

I have a strange issue with GtkGlArea under linux: it seems my program runs with a different fps than the monitor's refresh rate. (The rendering itself is fast, faster than 1ms.)

I use a 60Hz monitor mode, and the rendering fps is around 39.5Hz. I verified my monitor refresh rate with xrandr, and even with another -- X11-EGL based -- program; it is really 60Hz.

I also measured the time interval between two render calls, and the min and max is around 22ms and 28ms, so it seems that I even do not drop a frame.

I do not set anything special in GtkGlArea or opengl (glSwapInterval or so)...

Did you encounter with such an issue? Any idea what can be the problem?

Ubuntu 16.04, unity, gtk+ 3.18.9

Thanks,

Ferenc