Messages in this thread | | | Date | Fri, 25 Feb 2022 18:46:26 +0800 | Subject | Re: [RFC PATCH 0/5] introduce sched-idle balancing | From | Abel Wu <> |
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On 2/25/22 4:29 PM, Vincent Guittot Wrote: > On Fri, 25 Feb 2022 at 07:46, Abel Wu <wuyun.abel@bytedance.com> wrote: >> >> Hi Peter, >> >> On 2/24/22 11:20 PM, Peter Zijlstra Wrote: >>> On Thu, Feb 17, 2022 at 11:43:56PM +0800, Abel Wu wrote: >>>> Current load balancing is mainly based on cpu capacity >>>> and task util, which makes sense in the POV of overall >>>> throughput. While there still might be some improvement >>>> can be done by reducing number of overloaded cfs rqs if >>>> sched-idle or idle rq exists. >>> >>> I'm much confused, there is an explicit new-idle balancer and a periodic >>> idle balancer already there. >> >> The two balancers are triggered on the rqs that have no tasks on them, >> and load_balance() seems don't show a preference for non-idle tasks so > > The load balance will happen at the idle pace if a sched_idle task is > running on the cpu so you will have an ILB on each cpu that run a > sched-idle task
I'm afraid I don't quite follow you, since sched-idle balancer doesn't touch the ILB part, can you elaborate on this? Thanks.
> >> there might be possibility that only idle tasks are pulled during load >> balance while overloaded rqs (rq->cfs.h_nr_running > 1) exist. As a > > There is a LB_MIN feature (disable by default) that filters task with > very low load ( < 16) which includes sched-idle task which has a max > load of 3
This feature might not that friendly to the situation that only sched-idle tasks are running in the system. And this situation can last more than half a day in our co-location systems in which the training/batch tasks are placed under idle groups or directly assigned to SCHED_IDLE.
> >> result the normal tasks, mostly latency-critical ones in our case, on >> that overloaded rq still suffer waiting for each other. I observed this >> through perf sched. >> >> IOW the main difference from the POV of load_balance() between the >> latency-critical tasks and the idle ones is load. >> >> The sched-idle balancer is triggered on the sched-idle rqs periodically >> and the newly-idle ones. It does a 'fast' pull of non-idle tasks from >> the overloaded rqs to the sched-idle/idle ones to let the non-idle tasks >> make full use of cpu resources. >> >> The sched-idle balancer only focuses on non-idle tasks' performance, so >> it can introduce overall load imbalance, and that's why I put it before >> load_balance(). > > According to the very low weight of a sched-idle task, I don't expect > much imbalance because of sched-idle tasks. But this also depends of > the number of sched-idle task. > > >> >> Best Regards, >> Abel
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