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

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

On 08/15/2012 10:43 PM, Peter Zijlstra wrote:

> On Wed, 2012-08-15 at 15:15 +0200, 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?
> 
> Sure we can do that.
> 
>>> 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.
>>
>> 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...
> 
> I was thinking of a utilization measure made of per-task weighted
> runnable averages. This should indeed cover that case and we'll overflow
> when on average there is no (significant) idle time over a period longer
> than the averaging period.


It's also a good idea. :)

> 
> Anyway, I'm not too set on this and I'm very sure we can tweak this ad
> infinitum, so starting with something relatively simple that works for
> most is preferred.
> 
> As already stated, I think some of the Linaro people actually played
> around with something like this based on PJTs patches.


Vincent, would you like to introduce more?