DRI3 is not the default setting (Skylake GT2)
While trying to work out an issue with Mesa 20.0.0 with iris using the intel display drivers mesa/mesa#2569 (closed) I discovered that it appears that DRI 3 is not being used by my Mesa Intel(R) HD Graphics 530 (SKL GT2).
I tested an intensive video game process running simultaneously the same cut scene over and over in a controlled environment while glxgears is running.
Here are the results of this test in i965 environment:
5747.7040877193 FramesPerSecond [blank - i.e. no dri setting specified (default)]
5968.33305263158 FramesPerSecond Option "DRI" "3"
5717.58489473684 FramesPerSecond Option "DRI" "true"
5710.27394736842 FramesPerSecond Option "DRI" "2"
514.665578947368 FramesPerSecond Option "DRI" "1" (not a typo ~500 lol)
515.992 FramesPerSecond Option "DRI" "false"
528.772 FramesPerSecond Option "DRI" "off"
5550.83156140351 FramesPerSecond Option "DRI" "none" (appears like it reverts to default i.e. what looks like DRI 2)
514.397 FPS FramesPerSecond Option "DRI" "0"
Everywhere you search on the internet they say that DRI3 is the default behavior for xf86-video-intel drivers. Clearly, something with my setup suggests that isn't the case. As soon as I explicitly wrote the option "DRI" "3" the artifacts and text corruption on Mesa 20.0.0 with iris went away as well.
Is it a known thing that on some older cards Ubuntu (or some other setting, xf86-video-intel, etc) uses DRI 2 in place of 3 as the default? Using DRI2 is a huge performance drop! Who can solve this? What can we do?