Re: [PATCH] O17int

Con Kolivas
2003-08-21T23:59:34Z

Quoting Mike Galbraith <efault@gmx.de>:

> At 09:46 PM 8/21/2003 +1000, Con Kolivas wrote:
> >On Thu, 21 Aug 2003 17:53, Mike Galbraith wrote:
> > > At 03:26 PM 8/21/2003 +1000, Con Kolivas wrote:
> > > >Unhappy with this latest O16.3-O17int patch I'm withdrawing it, and
> > > >recommending nothing on top of O16.3 yet.
> > > >
> > > >More and more it just seems to be a bandaid to the priority inverting
> spin
> > > > on waiters, and it does seem to be of detriment to general
> interacivity.
> > > > I can now reproduce some loss of interactive feel with O17.
> > > >
> > > >Something specific for the spin on waiters is required that doesn't
> affect
> > > >general performance. The fact that I can reproduce the same starvation
> in
> > > >vanilla 2.6.0-test3 but to a lesser extent means this is an intrinsic
> > > > problem that needs a specific solution.
> > >
> > > I can see only one possible answer to this - never allow a normal task
> to
> > > hold the cpu for long stretches (define) while there are other tasks
> > > runnable. (see attachment)
> >
> >I assume you mean the strace ? That was the only attachment, and it just
> >looks
> >like shiploads of schedule() from the get time of day. Yes?
>
> (no, ~2 seconds of X being awol)
>
> > > I think the _easiest_ fix for this particular starvation (without
> tossing
> > > baby out with bath water;) is to try to "down-shift" in schedule() when
> > > next == prev. This you can do very cheaply with a find_next_bit().
> That
> > > won't help the case where there are multiple tasks involved, but should
> fix
> > > the most common case for dirt cheap. (another simple alternative is to
> > > globally "down-shift" periodically)
> >
> >Err funny you should say that; that's what O17 did. But it hurt because it
> >would never allow a task that used a full timeslice to be next==prev. The
>
> If someone is marked for resched, it means we want to give someone else the
> cpu right? In this case at least, re-selecting blender is not the right
> thing to do. Looks like he's burning rubber... going nowhere fast.
>
> > less I throttled that, the less effective the antistarvation was. However
> >this is clearly a problem without using up full timeslices. I originally
> >thought they weren't trying to schedule lots because of the drop in ctx
> >during starvation but I forgot that rescheduling the same task doesnt count
> >as a ctx.
>
> Hmm. In what way did it hurt interactivity? I know that if you pass the
> cpu off to non-gui task who's going to use his full 100 ms slice, you'll
> definitely feel it. (made workaround, will spare delicate tummies;) If

Well I made it so that if full timeslice is used, and then the same task is
next==prev, let next candidate for scheduling get up to TIMESLICE_GRANULARITY
timeslice; not a full task timeslice. Even this was palpable as losing the X
waving around smoothness with anything else burning (kernel compile, bunzip or
whatever). Basically X is somewhere between a frequent sleeper with short
periods of cpu time, or a burst of full timeslices when used heavily. When it's
full timeslices, it suffers under the same things that throttle spinners.

> you mean that, say X releases the cpu and has only a couple of ms left on
> it's slice and is alone in it's queue, that preempting it at the end of
> it's slice after having only had the cpu for such a short time after wakeup
> hurts, you can qualify the preempt decision with a cpu possession time
> check.

I was letting it run out the full timeslice unabated, and only preventing it
from getting immediately rescheduled.

>
> > Also I recall that winegames got much better in O10 when everything was
> >charged at least one jiffy (pre nanosecond timing) suggesting those that
> were
> >waking up for minute amounts of time repeatedly were being penalised; thus
> >taking out the possibility of the starving task staying high priority for
> >long.
>
> (unsure what you mean here)

Ok I think blender (we say blenndah in .au) is waking up, polling for X, then
going back to sleep only to do it all over again. I don't think it's wasting a
full timeslice at all. There seem to be two variants of this spin on wait; one
is the task that uses a full timeslice spinning (wine games), and the other is
waking up, rescheduling and going back to sleep only to do it all over again.

>
> > > The most generally effective form of the "down-shift" anti-starvation
> > > tactic that I've tried, is to periodically check the head of all queues
> > > below current position (can do very quickly), and actively select the
> > > oldest task who hasn't run since some defined deadline. Queues are
> > > serviced based upon priority most of the time, and based upon age some
> of
> > > the time.
> >
> >Hmm also sounds fudgy.
>
> Yeah. I crossbred it with a ~deadline scheduler, and created a mutt.

But how did this mutt perform?

> -Mike (2'6"")

+ another 2'6"" doesn't quite get us tall enough. Maybe we need another
munchkin.

Con