[RFC PATCH v3 5/6] sched: pack the idle load balance

[alex.shi@intel.com (Alex Shi)]

On 03/27/2013 06:30 PM, Vincent Guittot wrote:
>> Arguing the performance/power balance does no much sense without
>> > detailed scenario. We just want to seek a flexible compromise way.
>> > But fixed buddy cpu is not flexible. and it may lose many possible
>> > powersaving fit scenarios on x86 system. Like if 2 SMT cpu can handle
>> > all tasks, we don't need to wake another core. or if 2 cores in one
>> > socket can handle tasks, we also don't need to wakeup another socket.
> Using 2 SMT for all tasks implies to accept latency and to share
> resources like cache and memory bandwidth so it means that you also
> accept some potential performance decrease which implies that someone
> must select this mode with a knob.
> The primary goal of the patchset is not to select between powersaving
> and performance but to stay in performance mode. We pack the small
> tasks in one CPU so the performance will not decrease but the low load
> scenario will consume less power. Then, I can add another step which
> will be more power saving aggressive with a potential cost of
> performance and i this case the buddy CPU will be updated dynamically
> according to the system load
> 

Predication of small task behavior is often wrong. so for performance
purpose, packing task is a bad idea.

-- 
Thanks
    Alex