I’ve been suffering from the notorious problem of 100% CPU usage and adventitious window freeze or even the whole system collapse from Mozilla Firefox for a long time. The detailed phenomenon is that a process named “Web Content” fully occupies the CPU and there are also error messages in the console as below.

nouveau: kernel rejected pushbuf: Invalid argument
nouveau: ch18: krec 0 pushes 1 bufs 10 relocs 0
nouveau: ch18: buf 00000000 00000015 00000002 00000002 00000000
nouveau: ch18: buf 00000001 00000007 00000002 00000002 00000000
nouveau: ch18: buf 00000002 0000000a 00000002 00000002 00000000
nouveau: ch18: buf 00000003 0000000b 00000002 00000002 00000000
nouveau: ch18: buf 00000004 00000008 00000002 00000002 00000000
nouveau: ch18: buf 00000005 00000006 00000004 00000000 00000004
nouveau: ch18: buf 00000006 0000001e 00000002 00000000 00000002
nouveau: ch18: buf 00000007 00000020 00000002 00000000 00000002
nouveau: ch18: buf 00000008 00000024 00000002 00000000 00000002
nouveau: ch18: buf 00000009 00000030 00000002 00000000 00000002
nouveau: ch18: psh 00000032 000007715c 0000077d68

Initially, I thought this might be caused by Firefox’s own multi-threading mechanism. So I tried to limit the number of processes used by Firefox, but it did not work.

Another guess on the cause of the problem might be related to the open source display driver nouveau. After some search, I came to this bug report, which suggests to set the environment variable WEBKIT_DISABLE_COMPOSITING_MODE=1. According to the documentation of WebKit, this option forces the accelerated compositing mode adopted by WebKitGTK to be always off. And this just solves my problem.