Today Phoronix published (again) test results comparing the Game Rendering Performance on various desktop environments. As it “seems” like the performance of Game Rendering under KWin got worse in comparison to last year I want to point out a few things.
First of all: keep in mind that this is a test of Game Rendering Performance and doesn’t tell anything about the performance of tasks that matter. The development focus of KWin is clearly not on being great for games. We want to be (and are) the best composited window manager for desktop systems. That is what matters that is what we fine tune the system for.
Another important fact to know is that last years test was seriously flawed and I think this years test is flawed in a similar way. Last year KWin used by default unredirection of fullscreen windows while no other composited window manager in the test did that. With 4.7 we adjusted the default and turned unredirection off. But at the same time Mutter received to my knowledge unredirection of fullscreen windows. In case it is enabled in Mutter we have the explanation for last years bad results and this years good results.
If I would perform such a test, I would not benchmark KWin once but once for OpenGL 2.x backend, once for OpenGL ES 2.0 backend, once for OpenGL 1.x backend, once for XRender backend, the same set with unredirection of fullscreen windows turned on and once without compositing at all. We see there are so many things that would influence the game rendering performance that just one run of the benchmark is not enough.
But still we would recommend to turn compositing off when playing games. At least that is what I would do. A window manager is mostly just in the way of games and that is one of the reasons why gaming on desktop is by far not as good as playing on a gaming console. So if you want to game with KWin use the feature to specify a window specific rule to block compositing as long as the game is running. This will yield the best game rendering performance.