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

[Mike Galbraith <efault@gmx.de>]

At 10:13 AM 1/6/2006 +1100, Peter Williams wrote:
> 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.
Color me skeptical, but nonetheless, it looks to me like the mechanism 
might need the attached.
On the subject of nip and tuck, take a look at the little proggy posted in 
thread [SCHED] wrong priority calc - SIMPLE test case.  That testcase was 
the result of Paolo Ornati looking into a real problem on his system.  I 
just 'fixed' that nanosleep() problem by judicious application of 
TASK_NONINTERACTIVE to the schedule_timeout().  Sure, it works, but it 
doesn't look like anything but a bandaid (tourniquet in this case:) to me.
        -Mike

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