Re: sched: Consequences of integrating the Per Entity Load Tracking Metric into the Load Balancer

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

On 01/09/2013 11:14 AM, Preeti U Murthy wrote:
>>>>> Here comes the point of making both load balancing and wake up
>>>>> balance(select_idle_sibling) co operative. How about we always schedule
>>>>> the woken up task on the prev_cpu? This seems more sensible considering
>>>>> load balancing considers blocked load as being a part of the load of cpu2.
>>>>
>>>> Hi Preeti,
>>>>
>>>> I'm not sure that we want such steady state at cores level because we
>>>> take advantage of migrating wake up tasks between cores that share
>>>> their cache as Matthew demonstrated. But I agree that reaching such
>>>> steady state at cluster and CPU level is interesting.
>>>>
>>>> IMHO, you're right that taking the blocked load into consideration
>>>> should minimize tasks migration between cluster but it should no
>>>> prevent fast task migration between cores that share their cache
>>>
>>> True Vincent.But I think the one disadvantage even at cpu or cluster
>>> level is that when we consider blocked load, we might prevent any more
>>> tasks from being scheduled on that cpu during periodic load balance if
>>> the blocked load is too much.This is very poor cpu utilization
>>
>> The blocked load of a cluster will be high if the blocked tasks have
>> run recently. The contribution of a blocked task will be divided by 2
>> each 32ms, so it means that a high blocked load will be made of recent
>> running tasks and the long sleeping tasks will not influence the load
>> balancing.
>> The load balance period is between 1 tick (10ms for idle load balance
>> on ARM) and up to 256 ms (for busy load balance) so a high blocked
>> load should imply some tasks that have run recently otherwise your
>> blocked load will be small and will not have a large influence on your
>> load balance

Just tried using cfs's runnable_load_avg + blocked_load_avg in
weighted_cpuload() with my v3 patchset, aim9 shared workfile testing
show the performance dropped 70% more on the NHM EP machine. :(