Re: [discussion]sched: a rough proposal to enable power saving in scheduler

[Alex Shi <alex.shi@intel.com>]

On 08/15/2012 09:15 PM, Borislav Petkov wrote:

> On Wed, Aug 15, 2012 at 01:05:38PM +0200, Peter Zijlstra wrote:
>> On Mon, 2012-08-13 at 20:21 +0800, Alex Shi wrote:
>>> Since there is no power saving consideration in scheduler CFS, I has a
>>> very rough idea for enabling a new power saving schema in CFS.
>>
>> Adding Thomas, he always delights poking holes in power schemes.
>>
>>> It bases on the following assumption:
>>> 1, If there are many task crowd in system, just let few domain cpus
>>> running and let other cpus idle can not save power. Let all cpu take the
>>> load, finish tasks early, and then get into idle. will save more power
>>> and have better user experience.
>>
>> I'm not sure this is a valid assumption. I've had it explained to me by
>> various people that race-to-idle isn't always the best thing. It has to
>> do with the cost of switching power states and the duration of execution
>> and other such things.
> 
> I think what he means here is that we might want to let all cores on
> the node (i.e., domain) finish and then power down the whole node which
> should bring much more power savings than letting a subset of the cores
> idle. Alex?


Yes, that is my assumption. If my memory service me well. The idea get
from Suresh when introducing the old power saving schema.

> 
> [ … ]
> 
>> So I'd leave the currently implemented scheme as performance, and I
>> don't think the above describes the current state.
>>
>>> 			} else if (schedule policy == power)
>>> 				move tasks from busiest group to
>>> 				idlest group until busiest is just full
>>> 				of capacity.
>>> 				//the busiest group can balance
>>> 				//internally after next time LB,
>>
>> There's another thing we need to do, and that is collect tasks in a
>> minimal amount of power domains.
> 
> Yep.
> 
> Btw, what heuristic would tell here when a domain overflows and another
> needs to get woken? Combined load of the whole domain?
> 
> And if I absolutely positively don't want a node to wake up, do I
> hotplug its cores off or are we going to have a way to tell the
> scheduler to overcommit the non-idle domains and spread the tasks only
> among them.


You are right. here using the least load non-idle group is better than
idlest.

> 
> I'm thinking of short bursts here where it would be probably beneficial
> to let the tasks rather wait runnable for a while then wake up the next
> node and waste power...


True. Maybe that is Peter mentioned '2*capacity' reason?

> 
> Thanks.
>