Re: [rfc patch] sched/fair: Use instantaneous load for fork/exec balancing

Matt Fleming
2016-07-06T11:45:48Z

On Mon, 04 Jul, at 07:43:14PM, Mike Galbraith wrote:
> On Mon, 2016-07-04 at 16:04 +0100, Matt Fleming wrote:
>
> > But we can optimise the special case of dequeueing the last entity and
> > reset ::runnable_load_avg early, which gives a performance improvement
> > to workloads that trigger the load balancer, such as fork-heavy
> > applications when SD_BALANCE_FORK is set, because it gives a more up
> > to date view of how busy the cpu is.
>
> Begs the question: what's so special about this case vs any other
> dequeue/enqueue?

All that makes this special is that this is the behaviour seen when
running hackbench - initial heavy forking by some master task which
eventually wakes everyone up. So you get this huge sequence of "fork,
enqueue, run, dequeue". Yes, it's a complete hack.

> I've given up on this as being a waste of time. Either you serialize
> everything box wide (not!) and can then make truly accurate evaluations
> of state, or you're making an educated guess based upon what once was.
>
> The only place I've seen where using the average consistently has
> issues is with a longish period periodic load (schbench).

I'm open to any suggestion that restores performance to that seen
before commit 0905f04eb21f, whether or not that involves changing how
load averages are used.