QAbstractEventDispatcher blocks and waits constantly on every external event
processed; every timer or update from an X or wayland client, mouse
move or DRM event.
Right now every time this happens we go and check Xwayland for new
events, this is a system call (poll) that based on strace will
unsurprisingly immediately return with EAGAIN as there's nothing to read
from X. If there is something to read our socket notifier will fire. On block we do still need to read any events read in the meantime that weren't dispatched.
This cuts down our system calls significantly, which hopefully should have a
noticeable impact on performance especially when the kernel is under
load.
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Found whilst analysing strace (by accident whilst looking for something else!)
In a simple case of xwayland nested running glxgears we go from 28 calls per frame to 21. With many many clients and more input events it'll be an even higher percentage.
This installs a socket notifier onto our xwayland socket, when a user
connects we launch xwayland. The client then connections once kwin has
established itself as the compositor.
For a full desktop plasma session this patch effectively does nothing
too useful as we still start kcminit and make xrdb calls on startup
which in turn will launch X, but for the same reason this patch should
be harmless now as we're still processing the xrdb calls before any
clients will connect.
It's somewhat popular for voice communication applications to support
Push-to-Talk. This means that the process itself expects to get all of
the system input. This behaviour albeit sound does not work on Wayland
systems.
This commit adds an option to let legacy X11 applications that assume
they will be getting all information to do so until these apps are
properly ported to the XDP GlobalShortcuts.
We gain nothing with it. XCB setup logic in the Xwayland server has to
be moved to the workspace layer anyway. For example, this move of
responsibilities will be needed to support running more than just one
instance of Xwayland. Architecture-wise, it would be cleaner too.
Unfortunately, it breaks encapsulation of the Application, but this can
be taken care later.
