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

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Date:	Sun, 20 Jan 2013 23:30:57 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	Vincent Guittot <vincent.guittot@...aro.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Paul McKenney <paul.mckenney@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	Venki Pallipadi <venki@...gle.com>,
	Robin Randhawa <robin.randhawa@....com>,
	Lists linaro-dev <linaro-dev@...ts.linaro.org>
Subject: Re: sched: Consequences of integrating the Per Entity Load Tracking
 Metric into the Load Balancer

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. :(
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