Re: [PATCH v3 16/22] sched: add power aware scheduling in fork/exec/wake

[Alex Shi]

On 01/17/2013 01:47 PM, Namhyung Kim wrote:
> On Wed, 16 Jan 2013 14:27:30 +0000, Morten Rasmussen wrote:
>> On Wed, Jan 16, 2013 at 06:02:21AM +0000, Alex Shi wrote:
>>> On 01/15/2013 12:09 AM, Morten Rasmussen wrote:
>>>> On Fri, Jan 11, 2013 at 07:08:45AM +0000, Alex Shi wrote:
>>>>> On 01/10/2013 11:01 PM, Morten Rasmussen wrote:
>>>>> For power consideration scenario, it ask task number less than Lcpu
>>>>> number, don't care the load weight, since whatever the load weight, the
>>>>> task only can burn one LCPU.
>>>>>
>>>>
>>>> True, but you miss the opportunities for power saving when you have many
>>>> light tasks (> LCPU). Currently, the sd_utils < threshold check will go
>>>> for SCHED_POLICY_PERFORMANCE if the number tasks (sd_utils) is greater
>>>> than the domain weight/capacity irrespective of the actual load caused
>>>> by those tasks.
>>>>
>>>> If you used tracked task load weight for sd_utils instead you would be
>>>> able to go for power saving in scenarios with many light tasks as well.
>>>
>>> yes, that's right on power consideration. but for performance consider,
>>> it's better to spread tasks on different LCPU to save CS cost. And if
>>> the cpu usage is nearly full, we don't know if some tasks real want more
>>> cpu time.
>>
>> If the cpu is nearly full according to its tracked load it should not be
>> used for packing more tasks. It is the nearly idle scenario that I am
>> more interested in. If you have lots of task with tracked load <10% then
>> why not pack them. The performance impact should be minimal.

I had tried using runnable utils with many methods, include similar  way
in late regular balance. But like I talked with Mike before, the burst
waking up has no time to accumulate util, so tasks were set to few cpus
and than pulled away in regular balance, that cause many performance
benchmark drop much.

So I'd rather assume very new task will keep busy. If it is not, we
still can pull them in regular balance.
>>
>> Furthermore, nr_running is just a snapshot of the current runqueue
>> status. The combination of runnable and blocked load should give a
>> better overall view of the cpu loads.
> 
> I have a feeling that power aware scheduling policy has to deal only
> with the utilization.  Of course it only works under a certain threshold
> and if it's exceeded must be changed to other policy which cares the
> load weight/average.  Just throwing an idea. :)
> 
>>
>>> Even in the power sched policy, we still want to get better performance
>>> if it's possible. :)
>>
>> I agree if it comes for free in terms of power. In my opinion it is
>> acceptable to sacrifice a bit of performance to save power when using a
>> power sched policy as long as the performance regression can be
>> justified by the power savings. It will of course depend on the system
>> and its usage how trade-off power and performance. My point is just that
>> with multiple sched policies (performance, balance and power as you
>> propose) it should be acceptable to focus on power for the power policy
>> and let users that only/mostly care about performance use the balance or
>> performance policy.
> 
> Agreed.
> 

Firstly I hope the 'balance' policy can be used on widely on sever,
thus, it's better not to hurt performance.

Secondly 'race to idle' is one of the patchset's assumption, if we can
finish the tasks more early. we can save more power.

Last but not least, if the patch is merged, we can do more tunning on
'power' policy. :)
>>>>
>>>> Thanks for clarifying. To the best of my knowledge there are no
>>>> guidelines for how to specify cpu power so it may be a bit dangerous to
>>>> assume that capacity < weight when capacity is based on cpu power.
>>>
>>> Sure. I also just got them from code. and don't know other arch how to
>>> different them.
>>> but currently, seems this cpu power concept works fine.
>>
>> Yes, it seems to work fine for your test platform. I just want to
>> highlight that the assumption you make might not be valid for other
>> architectures. I know that cpu power is not widely used, but that may
>> change with the increasing focus on power aware scheduling.

cpu_power defined and used in general code. I saw arm and powerpc
mentioned them much in self arch code.

Anyway, would you like to share which arch doesn't fit this?
> 
> AFAIK on ARM big.LITTLE, a big cpu will have a cpu power more than
> 1024.  I'm sure Morten knows way more than me on this. :)
> 
> Thanks,
> Namhyung
> 


-- 
Thanks
    Alex
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