Subject: [patch 0/2] sLeAZY FPU feature
Newsgroups: gmane.linux.kernel
Date: 2006-07-01 17:11:33 GMT (3 years, 3 days, 7 hours and 13 minutes ago)
Hi, the two patches in this series (the x86-64 on by me, the i386 one by Chuck Ebbert) change how the lazy fpu feature works. In the current situation, we are 100% lazy, meaning that after every context switch, the application takes a trap on the first FPU use, which then restores the FPU context. The sLeAZY FPU patch changes this behavior; if a process has used the FPU for 5 stints at a row, the behavior becomes proactive and the FPU context is restored during the regular context switch already. This means we can avoid the trap. The underlying assumption is that if a process uses 5 times consecutive, it's likely to do it the 6th and later times as well (eg it's not a one-off behavior). There is a limit built in; this proactive behavior resets after 255 times, so that when a process is long lived and chances behavior, it'll still get the right behavior (for performance) after some time. Chuck measured a +/- 0.4% performance gain, and my experiments show a similar improvement. Greetings, Arjan van de Ven