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

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Date:	Thu, 26 Feb 2009 16:45:26 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Dipankar Sarma <dipankar@...ibm.com>
Cc:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	a.p.zijlstra@...llo.nl, ego@...ibm.com, tglx@...utronix.de,
	andi@...stfloor.org, venkatesh.pallipadi@...el.com,
	vatsa@...ux.vnet.ibm.com, arun@...ux.vnet.ibm.com,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC PATCH 0/4] timers: framework for migration between CPU


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

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