linux-kernel - Re: weakness of runnable load tracking?

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Date:	Thu, 06 Dec 2012 16:06:25 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Mike Galbraith <bitbucket@...ine.de>
CC:	Alex Shi <lkml.alex@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: weakness of runnable load tracking?

>>
>> Hi Paul & Ingo:
>>
>> In a short word of this issue: burst forking/waking tasks have no time
>> accumulate the load contribute, their runnable load are taken as zero.
>> that make select_task_rq do a wrong decision on which group is idlest.
> 
> As you pointed out above, new tasks can (and imho should) be born with
> full weight.  Tasks _may_ become thin, but they're all born hungry.

Thanks for comments. I think so. :)
> 
>> There is still 3 kinds of solution is helpful for this issue.
>>
>> a, set a unzero minimum value for the long time sleeping task. but it
>> seems unfair for other tasks these just sleep a short while.
>>
>> b, just use runnable load contrib in load balance. Still using
>> nr_running to judge idlest group in select_task_rq_fair. but that may
>> cause a bit more migrations in future load balance.
>>
>> c, consider both runnable load and nr_running in the group: like in the
>> searching domain, the nr_running number increased a certain number, like
>> double of the domain span, in a certain time. we will think it's a burst
>> forking/waking happened, then just count the nr_running as the idlest
>> group criteria.
>>
>> IMHO, I like the 3rd one a bit more. as to the certain time to judge if
>> a burst happened, since we will calculate the runnable avg at very tick,
>> so if increased nr_running is beyond sd->span_weight in 2 ticks, means
>> burst happening. What's your opinion of this?
>>
>> Any comments are appreciated!
> 
> IMHO, for fork and bursty wake balancing, the only thing meaningful is
> the here and now state of runqueues tasks are being dumped into.
> 
> Just because tasks are historically short running, you don't necessarily
> want to take a gaggle and wedge them into a too small group just to even
> out load averages.  If there was a hole available that you passed up by
> using average load, you lose utilization.  I can see how this load
> tracking stuff can average out to a win on a ~heavily loaded box, but
> bursty stuff I don't see how it can do anything but harm, so imho, the
> user should choose which is best for his box, instantaneous or history.

Do you mean the system administrator need to do this choice?
It's may a hard decision.  :)
Any suggestions of decision basis?
> 
> WRT burst detection: any window you define can be longer than the burst.

Maybe we can define 2 waking on same cpu in 1 tick is a burst happened,
and if the cpu had taken a waking task. we'd better skip this cpu. :)
Anyway, the hard point is we can not predict future.


> 
> $.02
> 
> -Mike
> 

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