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

[Ingo Molnar <mingo@elte.hu>]

* Dipankar Sarma <dipankar@in.ibm.com> wrote:

> On Mon, Feb 23, 2009 at 12:07:25PM +0100, Ingo Molnar wrote:
> > 
> > * Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> 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