linux-kernel - Re: [RFC PATCH 0/5] enable runnable load avg in load balance

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Date:	Tue, 27 Nov 2012 14:14:02 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	Benjamin Segall <bsegall@...gle.com>, mingo@...hat.com,
	peterz@...radead.org, pjt@...gle.com, vincent.guittot@...aro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] enable runnable load avg in load balance

On 11/27/2012 11:08 AM, Preeti U Murthy wrote:
> Hi everyone,
> 
> On 11/27/2012 12:33 AM, Benjamin Segall wrote:
>> So, I've been trying out using the runnable averages for load balance in
>> a few ways, but haven't actually gotten any improvement on the
>> benchmarks I've run. I'll post my patches once I have the numbers down,
>> but it's generally been about half a percent to 1% worse on the tests
>> I've tried.
>>
>> The basic idea is to use (cfs_rq->runnable_load_avg +
>> cfs_rq->blocked_load_avg) (which should be equivalent to doing
>> load_avg_contrib on the rq) for cfs_rqs and possibly the rq, and
>> p->se.load.weight * p->se.avg.runnable_avg_sum / period for tasks.
> 
> Why should cfs_rq->blocked_load_avg be included to calculate the load
> on the rq? They do not contribute to the active load of the cpu right?
> 
> When a task goes to sleep its load is removed from cfs_rq->load.weight
> as well in account_entity_dequeue(). Which means the load balancer
> considers a sleeping entity as *not* contributing to the active runqueue
> load.So shouldn't the new metric consider cfs_rq->runnable_load_avg alone?
>>
>> I have not yet tried including wake_affine, so this has just involved
>> h_load (task_load_down and task_h_load), as that makes everything
>> (besides wake_affine) be based on either the new averages or the
>> rq->cpu_load averages.
>>
> 
> Yeah I have been trying to view the performance as well,but with
> cfs_rq->runnable_load_avg as the rq load contribution and the task load,
> same as mentioned above.I have not completed my experiments but I would
> expect some significant performance difference due to the below scenario:
> 
>                      Task3(10% task)
> Task1(100% task)     Task4(10% task)
> Task2(100% task)     Task5(10% task)
> ---------------     ----------------       ----------
> CPU1                  CPU2                  CPU3
> 
> When cpu3 triggers load balancing:
> 
> CASE1:
>  without PJT's metric the following loads will be perceived
>  CPU1->2048
>  CPU2->3042
>  Therefore CPU2 might be relieved of one task to result in:
> 
> 
> Task1(100% task)     Task4(10% task)
> Task2(100% task)     Task5(10% task)       Task3(10% task)
> ---------------     ----------------       ----------
> CPU1                  CPU2                  CPU3
> 
> CASE2:
>   with PJT's metric the following loads will be perceived
>   CPU1->2048
>   CPU2->1022
>  Therefore CPU1 might be relieved of one task to result in:
> 
>                      Task3(10% task)
>                      Task4(10% task)
> Task2(100% task)     Task5(10% task)     Task1(100% task)
> ---------------     ----------------       ----------
> CPU1                  CPU2                  CPU3
> 
> 
> The differences between the above two scenarios include:
> 
> 1.Reduced latency for Task1 in CASE2,which is the right task to be moved
> in the above scenario.
> 
> 2.Even though in the former case CPU2 is relieved of one task,its of no
> use if Task3 is going to sleep most of the time.This might result in
> more load balancing on behalf of cpu3.
> 
> What do you guys think?

It looks fine. just a question of CASE 1.
Usually the cpu2 with 3 10% load task will show nr_running == 0, at 70%
time. So, how you make rq->nr_running = 3 always?

Guess in most chance load balance with pull task1 or task2 to cpu2 or
cpu3. not the result of CASE 1.


> 
> Thank you
> 
> Regards
> Preeti U Murthy
> 
> 
> 
> 

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