Odd mouse behaviour

[plummew]

Anyone encountered this problem before:

My FPS is set at 60Hz and I have the usual update-render-display loop for 3D display.

My first person mouse-look works fine as long as the update-render phase takes a reasonable length of time. (Say 1/100th of a second). However, if the update-render phase takes less than 1/500th of a second the mouse-look becomes very slow and erratic.

As OpenGL is vsync-ing to 60Hz, the actual FPS never changes, so I'm puzzled as to what is going on.

Anyone got any ideas?

By the way - The obvious (but crude) solution to this is to run a delay loop if the update-render is too quick but it would be nice to understand what is causing this problem.