Re: [PATCH 0/4] Really lazy fpu

[George Spelvin]

> But on busy servers where most wakeups are IRQ based the chance of being on 
> the right CPU is 1/nr_cpus - i.e. decreasing with every new generation of 
> CPUs.

That doesn't seem right.  If the server is busy with FPU-using tasks, then
the FPU state has already been swapped out, and no IPI is necessary.

The worst-case seems to be a lot of non-FPU CPU hogs, and a few FPU-using tasks
that get bounced around the CPUs like pinballs.

It is an explicit scheduler goal to keep tasks on the same CPU across
schedules, so they get to re-use their cache state.  The IPI only happens
when that goal is not met, *and* the FPU state has not been forced out
by another FPU-using task.

Not completely trivial to arrange.


(An halfway version of this optimization whoch sould avoid the need for
an IPI would be *save* the FPU state, but mark it "clean", so the re-load
can be skipped if we're lucky.  If the code supported this as well as the
IPI alternative, you could make a heuristic guess at switch-out time
whether to save immediately or hope the odds of needing the IPI are less than
the fxsave/IPI cost ratio.)
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