Re: weakness of runnable load tracking?

From: Mike Galbraith
Date: Thu Dec 06 2012 - 04:12:20 EST

On Thu, 2012-12-06 at 16:06 +0800, Alex Shi wrote:
> >>
> >> 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?

That's my gut feeling just from pondering potential pitfalls.

> It's may a hard decision. :)

Yup, very hard.

> Any suggestions of decision basis?

Same as most buttons.. poke it and <cringe> see what happens :)

> > 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.

No matter what the metric, you'll be reacting after the fact.

Somebody needs to code up that darn omniscience algorithm. In a pinch,
a simple undo the past will suffice :)


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