So, first of all, this is all very much work-in-progress and highly experimental. It’s related to the work on screen management which I’ve outlined in an earlier article.
I ran a few benchmarks across our wayland stack, especially measuring interprocess communication performance when switching from X11 (or, in fact XCB and XRandR) to wayland. I haven’t done a highly scientific setup, just ran the same code with different backends to see how long it takes to receive information about screens connected, their modes, etc..
I also ran the numbers when loading the libkscreen backend in-process, more on that later.
The spreadsheet shows three data columns, in vertical blocks per backend the results for 4-5 individual runs and their mean values. One column for the default out-of-process mode, one with loading the backend in process and one showing the speedup between in- and out-of-process of the same backend.
The lower part contains some cross referencing of the mean values to compare different setups.
All values are nano seconds.
My results show a speedup of between 2 and 2.5 times when querying screen information on X11 and on wayland, wayland being much faster here.
The qscreen and xrandr backends perform pretty similar, they’re both going through XCB. That checks out. The difference between wayland and xrandr/qscreen can then be attributed to either the wayland protocol or its implementation in KWayland being much faster than the corresponding XCB implementations.
But, here’s the kicker…
in- vs. out-of-process
The main overhead, as it turns out, is libkscreen loading the backend plugins out-of-process. That means that it starts a dbus-autolaunched backend process and then passes data over DBus between the libkscreen front-end API and the backend plugin. It’s done that way to shield the client API (for example the plasma shell process or systemsettings) from unsafe calls into X11, as it encapsulates some crash-prone code in the XRandR backend. When using the wayland backend, this is not necessary, as we’re using KWayland, which is much safer.
I went ahead and instrumented libkscreen in a way that these backends are being loaded in process, which avoids most of the overhead. This change has an even more dramatic influence on performance: on X11, the speedup is 1.6x – 2x, on wayland loading the backend in-process makes it run 10 times faster. Of course, these speedups are complementary, so combined, querying screen information on wayland can be done about 20 times faster.
While this change from out-of-process to in-process backends introduces a bit more complexity in the library, it has a couple of other advantages additional to the performance gains. In-process means that debugging is much easier. If there are crashes, we do not hide them anymore, but identify and fix them. It also makes development more worthwhile, since it’s much easier to debug and test the backends and frontend API together. It also means that we can load backend plugins at the same time.
I’ve uploaded the benchmark data here. Before merging this, I’ll have to iron out some more wrinkles and have the code reviewed, so it’s not quite ready for prime-time yet.