Re: [PATCH] scheduler patch, faster still

Rik van Riel (H.H.vanRiel@phys.uu.nl)
Mon, 28 Sep 1998 20:26:41 +0200 (CEST)


On Mon, 28 Sep 1998 yodaiken@chelm.cs.nmt.edu wrote:
> On Mon, Sep 28, 1998 at 11:01:43AM +0200, Rik van Riel wrote:
> > > I must have missed these results. What I saw were microbenchmark results
> > > with no kernel profiling and no explanation. Why should searching a
> > > queue of 10 on 586 take any significant time? Where is the time spent?
> >
> > There is only one place where it _can_ be spent (the scheduler
> > is quite simple) and that is at scanning the run_queue.
>
> One of the reasons that Linux is such a delightful operating system is
> that your type of "reasoning" has usually been ignored.

Come on, we've all seen the code. Scanning the longer run
queue is the _only_ thing that's done when you have a
long run queue. If you've read the code, you'll reach
the same conclusion I reached...

> > > I'm very skeptical of loose use of the term RT. Are these tasks of yours
> > > really RT? Hard? Soft? Statistical?
> >
> > They are fairly hard and are triggered from an interrupt. After
>
> "Fairly hard" means absolutely nothing at all.

Yes it does. It means that the maximum latency is short.
Hard means nil latency and soft means we don't care that
much. The app in question is somewhere in between, but
closest to real hard RT.

> > > How? It's quite simple as it is.
> >
> > Reducing overall simplicity is not the goal. The main goal
>
> Coulda fooled me.

OK, you tell me what's better:
1) a simple system with a fair worst-case latency for RT tasks
2) a slightly more complex system with excellent worst-case
latency for RT tasks and a slightly worst latency for non-RT
tasks
3) a horribly complex and slow system, which nobody is proposing
since we can buy NT if we want one of these :)

Rik.
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