Arjan van de Ven wrote:
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.
What sort of test? Any idea of the results for a best case microbenchmark
(something like two threads ping-pong a couple of futexes between them,
in between doing a single FPU op)
--
SUSE Labs, Novell Inc.
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