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Subject: Re: [RFC][PATCH v5 00/14] sched: packing tasks
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From: Catalin Marinas <catalin.marinas@arm.com>
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Date: Tue, 12 Nov 2013 23:14:05 +0000
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To: Arjan van de Ven <arjan@linux.intel.com>
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On 12 Nov 2013, at 16:48, Arjan van de Ven <arjan@linux.intel.com> wrote:
> On 11/11/2013 10:18 AM, Catalin Marinas wrote:
>> The ordering is based on the actual C-state, so a simple way is to wake
>> up the CPU in the shallowest C-state. With asymmetric configurations
>> (big.LITTLE) we have different costs for the same C-state, so this would
>> come in handy.
> 
> btw I was considering something else; in practice CPUs will be in the deepest state..
> ... at which point I was going to go with some other metrics of what is best from a platform level

I agree, other metrics are needed.  The problem is that we currently
only have (relatively, guessed from the target residency) the cost of
transition from a C-state to a P-state (for the latter, not sure which).
But we don’t know what the power (saving) on that C-state is nor the one 
at a P-state (and vendors reluctant to provide such information). So the 
best the scheduler can do is optimise the wake-up cost and blindly assume 
that deeper C-state on a CPU is more efficient than lower P-states on two 
other CPUs (or the other way around).

If we find a good use for such metrics in the scheduler, I think the
vendors would be more open to providing at least some relative (rather
than absolute) numbers.

Catalin