Diffusion KWin 22a441e07151

[platforms/x11] Force glXSwapBuffers to block with NVIDIA driver

Authored by ekurzinger on Mar 20 2019, 4:50 PM.


[platforms/x11] Force glXSwapBuffers to block with NVIDIA driver

The NVIDIA implementation of glXSwapBuffers will, by default, queue up
to two frames for presentation before blocking. KWin's compositor,
however, assumes that calls to glXSwapBuffers will always block until
the next vblank when rendering double buffered. This assumption isn't
valid, as glXSwapBuffers is specified as being an implicit glFlush,
not an implicit glFinish, and so it isn't required to block. When this
assumption is violated, KWin's frame timing logic will
break. Specifically, there will be extraneous calls to
setCompositeTimer with a waitTime of 0 after the non-blocking buffer
swaps, dramatically reducing desktop responsiveness. To remedy this,
a call to glXWaitGL was added by Thomas Luebking after glXSwapBuffers
in 2015 (see bug 346275, commit
8bea96d7018d02dff9462326ca9456f48e9fe9fb). That glXWaitGL call is
equivalent to a glFinish call in direct rendering, so it was a good
way to make glXSwapBuffers behave as though it implied a glFinish

However, the NVIDIA driver will by default do a busy wait in glFinish,
for reduced latency. Therefore that change dramatically increased CPU
usage. GL_YIELD can be set to USLEEP (case insensitive) to change
the behavior and use usleep instead. When using the NVIDIA driver,
KWin will disable vsync entirely if GL_YIELD isn't set to USLEEP
(case sensitive, a bug in KWin).

However, the NVIDIA driver supports another environment variable,
__GL_MaxFramesAllowed, which can be used to control how many frames
may be queued by glXSwapBuffers. If this is set to 1 the function
will always block until retrace, in line with KWin's expectations.
This allows the now-unnecessary call to glXWaitGL to be removed along
with the logic to conditionally disable vsync, providing a better
experience on NVIDIA hardware.

Reviewers: KWin, davidedmundson, zzag

Reviewed By: KWin, davidedmundson, zzag

Subscribers: kwin, davidedmundson, zzag

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19867


ekurzingerMar 20 2019, 4:56 PM
Differential Revision
D19867: [X11] Force glXSwapBuffers to block with NVIDIA driver
R108:d2820bf05eff: Don't try to resize desktop or fullscreen windows