Re: [patch 0/2] sLeAZY FPU feature

[Nick Piggin]

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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