Re: [PATCH 0/4] Really lazy fpu

[Avi Kivity <avi@redhat.com>]

On 06/13/2010 06:03 PM, Avi Kivity wrote:
> Currently fpu management is only lazy in one direction.  When we switch into
> a task, we may avoid loading the fpu state in the hope that the task will
> never use it.  If we guess right we save an fpu load/save cycle; if not,
> a Device not Available exception will remind us to load the fpu.
>
> However, in the other direction, fpu management is eager.  When we switch out
> of an fpu-using task, we always save its fpu state.
>
> This is wasteful if the task(s) that run until we switch back in all don't use
> the fpu, since we could have kept the task's fpu on the cpu all this time
> and saved an fpu save/load cycle.  This can be quite common with threaded
> interrupts, but will also happen with normal kernel threads and even normal
> user tasks.
>
> This patch series converts task fpu management to be fully lazy.  When
> switching out of a task, we keep its fpu state on the cpu, only flushing it
> if some other task needs the fpu.
>    

Ingo, Peter, any feedback on this?

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