Package: kwin-x11 Version: 4:5.14.5-1 Severity: important Tags: buster sid patch upstream fixed-upstream Forwarded: https://bugs.kde.org/show_bug.cgi?id=322060
kwin-x11 with the current nvidia driver causes extreme high CPU load and tearing. The graphics performance is also reduced to an unacceptable level when compositing is enabled. This is caused by the way how kwin-x11 tries to flush frames - which doesn't work with advanced drivers (like the ones from nvidia). This was better described by Erik Kurzinger in his patch https://phabricator.kde.org/D19867 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 call. 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. The patch can be found under https://phabricator.kde.org/D19867 and upstream (Martin Flöser) recommends to backport it to all older versions. This also includes Debian Buster (and maybe also Stretch - not sure about this part)
