linux-kernel - Re: [PATCH 0/4] Really lazy fpu

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Date:	16 Jun 2010 07:32:53 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	avi@...hat.com, mingo@...e.hu
Cc:	linux@...izon.com, linux-kernel@...r.kernel.org, npiggin@...e.de
Subject: Re: [PATCH 0/4] Really lazy fpu

> 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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