Re: [PATCH] sched: Fix adverse effects of NFS client on interactive response

[Peter Williams <pwil3058@bigpond.net.au>]

Mike Galbraith wrote:
> At 10:31 PM 1/5/2006 +1100, Peter Williams wrote:
>
> > Mike Galbraith wrote:
> >
> > > At 08:51 AM 1/5/2006 +1100, Peter Williams wrote:
> > >
> > > > I think that some of the harder to understand parts of the scheduler 
> > > > code are actually attempts to overcome the undesirable effects (such 
> > > > as those I've described) of inappropriately identifying tasks as 
> > > > interactive.  I think that it would have been better to attempt to 
> > > > fix the inappropriate identifications rather than their effects and 
> > > > I think the prudent use of TASK_NONINTERACTIVE is an important tool 
> > > > for achieving this.
> > >
> > >
> > > IMHO, that's nothing but a cover for the weaknesses induced by using 
> > > exclusively sleep time as an information source for the priority 
> > > calculation.  While this heuristic does work pretty darn well, it's 
> > > easily fooled (intentionally or otherwise).  The challenge is to find 
> > > the right low cost informational component, and to stir it in at O(1).
> >
> >
> > TASK_NONINTERACTIVE helps in this regard, is no cost in the code where 
> > it's used and probably decreases the costs in the scheduler code by 
> > enabling some processing to be skipped.  If by its judicious use the 
> > heuristic is only fed interactive sleep data the heuristics accuracy 
> > in identifying interactive tasks should be improved.  It may also 
> > allow the heuristic to be simplified.
>
>
> I disagree.  You can nip and tuck all the bits of sleep time you want, 
> and it'll just shift the lumpy spots around (btdt).

Yes, but there's a lot of (understandable) reluctance to do any major 
rework of this part of the scheduler so we're stuck with nips and tucks 
for the time being.  This patch is a zero cost nip and tuck.

If the plugsched patches were included in -mm we could get wider testing 
of alternative scheduling mechanisms.  But I think it will take a lot of 
testing of the new schedulers to allay fears that they may introduce new 
problems of their own.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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