Re: [PATCH 0/4] Really lazy fpu

Nick Piggin
2010-06-16T09:10:04Z

On Wed, Jun 16, 2010 at 10:39:41AM +0200, Ingo Molnar wrote:
>
> (Cc:-ed various performance/optimization folks)
>
> * Avi Kivity <avi@redhat.com> wrote:
>
> > On 06/16/2010 10:32 AM, H. Peter Anvin wrote:
> > >On 06/16/2010 12:24 AM, Avi Kivity wrote:
> > >>Ingo, Peter, any feedback on this?
> > > Conceptually, this makes sense to me. However, I have a concern what
> > > happens when a task is scheduled on another CPU, while its FPU state is
> > > still in registers in the original CPU. That would seem to require
> > > expensive IPIs to spill the state in order for the rescheduling to
> > > proceed, and this could really damage performance.
> >
> > Right, this optimization isn't free.
> >
> > I think the tradeoff is favourable since task migrations are much
> > less frequent than context switches within the same cpu, can the
> > scheduler experts comment?
>
> This cannot be stated categorically without precise measurements of
> known-good, known-bad, average FPU usage and average CPU usage scenarios. All
> these workloads have different characteristics.
>
> I can imagine bad effects across all sorts of workloads: tcpbench, AIM7,
> various lmbench components, X benchmarks, tiobench - you name it. Combined
> with the fact that most micro-benchmarks wont be using the FPU, while in the
> long run most processes will be using the FPU due to SIMM instructions. So
> even a positive result might be skewed in practice. Has to be measured
> carefully IMO - and i havent seen a _single_ performance measurement in the
> submission mail. This is really essential.

It can be nice to code an absolute worst-case microbenchmark too.

Task migration can actually be very important to the point of being
almost a fastpath in some workloads where threads are oversubscribed to
CPUs and blocking on some contented resource (IO or mutex or whatever).
I suspect the main issues in that case is the actual context switching
and contention, but it would be nice to see just how much slower it
could get.