glXWaitVideoSync sometimes gets throttled on XWayland

See the following Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1592786

Firefox needs vsync events to trigger its rendering pipeline. This is currently done on GLX by spawning a separate thread which has a loop with glXWaitVideoSync. This works perfectly on plain X11, however on XWayland it will randomly start falling back to 30fps on a 60fps monitor (missing every other event).

I thought the issue might be the renderer thread blocking on swapbuffers and holding a lock, however as far as I can tell there aren't any such locks.

added Xwayland label

My assumption has been that this is a mutter issue.

In contrast to what you wrote in https://bugzilla.mozilla.org/show_bug.cgi?id=1592786#c4, I'm seeing the MSC value only getting incremented by 1 every ~33 ms with affected apps (I'm using gravit & armagetronad for testing), which indicates that Xwayland already only gets one frame callback from mutter every ~33 ms.

The only way I could see this being an Xwayland issue would be if it somehow introduced significant delays between getting a callback from the Wayland server and sending events to the X client, or vice versa getting a PresentPixmap request from the X client and sending a surface commit to the Wayland server. But armagetronad runs at almost 1000 fps with vblank_mode=0 (in which case refresh is still driven by callbacks from the Wayland server, though different ones), so Xwayland can't be introducing more than ~1 ms of delay per round-trip.

Actually, in my case it turned out to be due to the GPU clocks staying low: drm/amd#1146 (closed)

Check which clock states / profiles are available on your system first, see https://dri.freedesktop.org/docs/drm/gpu/amdgpu.html#gpu-sysfs-power-state-interfaces for more information.

Closing this issue per above. Thanks for the report.

closed