Re: [SHED] Questions.

[Ian Kumlien <pomac@vapor.com>]

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On Sun, 2003-08-31 at 20:53, Robert Love wrote:
> On Sun, 2003-08-31 at 06:07, Ian Kumlien wrote:
> 
> > Why not use small quantum values for high pri processes and long for low
> > pri since the high pri processes will preempt the low pri processes
> > anyways. And for a server working under load with only a few processes
> > (assuming they are all low pri) would lessen the context switches.
> 
> The rationale behind giving high priority processes a large timeslice is
> two-fold:
> 
> (1) if they are interactive, then they won't actually use it all (this
> is the point you are making). But,
> 
> (2) Having a large timeslice ensures that they have a high probability
> of having available timeslice when they _do_ need it.

Since they would have a high pri still, and preempt is there... it
should be back on the cpu pretty quick.

> So, yes, interactive processes can get by with a small timeslice,
> because that is by-definition all they need.  But they do need to run
> often (i.e., as I think you have mentioned in your last email,
> interactive processes are "run often for short periods"), so the large
> timeslice ensures that they are never expired.

But, it also creates problems for when a interactive process becomes a
cpu hog. Like this the detection should be faster, but should be slowed
down somewhat.

> A counterargument might be that the large timeslice is a detriment to
> other high priority processes.  But the thinking is that, by definition,
> interactive processes won't use all of the timeslice.  And thus will not
> hog the CPU.  If they do, the interactivity estimator will quickly bring
> them down.

But, hogs would instead cause a context switch hell and lessen the
throughput on server loads...

> That is the rationale in the current scheduler, anyhow.  Nick's current
> work is interesting, and a bit different.

Yes, saner imho =)

> > And a system with "interactive load" as well would, as i said, preempt
> > the lower pris. But this could also cause a problem... Imho there should
> > be a "min quantum value" so that processes can't preempt a process that
> > was just scheduled (i dunno if this is implemented already though). 
> 
> I don't think this is a good idea.  I see your intention, but we have
> priorities for a reason.

I don't see how priorities would be questioned... Since, all i say is
that a task that gets preempted should have a guaranteed time on the cpu
so that we don't waste cycles doing context switches all the time. 

I can see that Ingos current scheduler is good from a desktop
standpoint, but having it that way is not warranted when preempt comes
in to the picture (if i correctly understand it's workings)... 
With preempt i actually see no reason for the priority inversion.. And
to answer someone who mailed about this before: "Yes, it does seem to be
slower than my Amigas, esp the ones that use Executive...".
(That feedback scheduler rocks =))

-- 
Ian Kumlien <pomac@vapor.com>

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