https://bugs.kde.org/show_bug.cgi?id=452219
--- Comment #7 from Stefan Hoffmeister <stefan.hoffmeis...@econos.de> ---
Let me provide some details, observations, and confirmations which may be
helpful (based on v5.24.5 / Fedora 36):
* Wayland
* Notebook with Optimus setup - Intel iGPU + Nvidia dGPU
* Intel GPU has (exclusive) control over the internal display and the HDMI port
("connector")
* Nvidia GPU has (exclusive) control over the USB-C output path (i.e.
DisplayPort Alternate Mode, Thunderbolt Alternative Mode) ("connector")
* Intel is the default (boot) GPU (and cannot be changed)
Log into an empty desktop and attach a 4K external screen to USB-C (which ends
up in a DisplayPort bitstream). The Nvidia GPU remains in "PRIME offload" mode,
but obviously needs the data for presenting.
Any rendering of data strictly towards the Intel-controlled connector remains
smooth. Any rendering towards the Nvidia-controlled connector results in
* an extremely jerky mouse cursor, everything is laggy
* massive CPU load from the kwin_wayland process in response to anything that
requires drawing, even just moving the mouse cursor around
It *feels* almost as if the complete content associated with the 4K screen at
large is transferred even, just for moving the mouse cursor.
Running 'perf trace' suggests that an enormous amount of time is spent on
memmove / memcpy (via glibc optimized functions); the attached screenshot shows
that (and right now I do not know how to produce better diagnostic data)
I tried installing debug symbols via the Fedora service; this got lines
resolved in glibc (the memcpy), but nothing for the KDE process (qt_metacast
smells like dynamic dispatch, so I am not all that surprised).
I turned on DRM logs
(https://invent.kde.org/plasma/kwin/-/wikis/Debugging-DRM-issues); there was
surprisingly few data, but not being familiar with the domain, I do not know
what to look for in those logs.
I can also confirm the observation that staying on the Intel GPU creates a
smooth experience: Disconnect the external screen from DisplayPort, attach it
to HDMI (Intel connector) - everything is smooth.
In closing, this is the Nvidia 510 driver, the most current driver available
for Fedora 36 from rpmfusion at the time of this writing.
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