Re: [RFC PATCH 0/4] timers: framework for migration between CPU

From: Ingo Molnar
Date: Thu Feb 26 2009 - 10:45:55 EST



* Dipankar Sarma <dipankar@xxxxxxxxxx> wrote:

> On Mon, Feb 23, 2009 at 12:07:25PM +0100, Ingo Molnar wrote:
> >
> > * Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > * Identify set of idle CPUs (CPU package) from which timers
> > > can be removed
> > > * Identify a semi-idle or idle CPU package to which the timers
> > > can be moved
> > > * Decide when to start moving timers as the system has large
> > > number of idle CPUs
> > > * Decide when to stop migrating as system becomes less idle
> > > and utilisation increases
> > >
> > > Guiding all of the above decisions from user space may not be
> > > fast enough.
> >
> > Exactly.
>
> That is true for power management. However there are other
> situations where we may need targeted avoidance of timers.
> Certain type of applications - HPC for example - prefer
> avoidance of jitters due to periodic timers. It would be good
> to be able to say "avoid these CPUs for timers" while they are
> being used for HPC tasks.

Yes - but that kind of policy should be coupled and expressed
via cpusets. /proc based irq_affinity is just a limited,
inflexible hack. All things IRQ partitioning should be handled
via cpusets - perhaps via the 'system sets' idea from Peter?

Ingo
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