Re: [ANNOUNCE] RSDL completely fair starvation free interactive cpuscheduler

[Bill Davidsen]

Con Kolivas wrote:
> On Monday 12 March 2007 22:26, Al Boldi wrote:
> > Con Kolivas wrote:
> > > On Monday 12 March 2007 15:42, Al Boldi wrote:
> > > > Con Kolivas wrote:
> > > > > On Monday 12 March 2007 08:52, Con Kolivas wrote:
> > > > > > And thank you! I think I know what's going on now. I think each
> > > > > > rotation is followed by another rotation before the higher priority
> > > > > > task is getting a look in in schedule() to even get quota and add
> > > > > > it to the runqueue quota. I'll try a simple change to see if that
> > > > > > helps. Patch coming up shortly.
> > > > > Can you try the following patch and see if it helps. There's also one
> > > > > minor preemption logic fix in there that I'm planning on including.
> > > > > Thanks!
> > > > Applied on top of v0.28 mainline, and there is no difference.
> > > >
> > > > What's it look like on your machine?
> > > The higher priority one always get 6-7ms whereas the lower priority one
> > > runs 6-7ms and then one larger perfectly bound expiration amount.
> > > Basically exactly as I'd expect. The higher priority task gets precisely
> > > RR_INTERVAL maximum latency whereas the lower priority task gets
> > > RR_INTERVAL min and full expiration (according to the virtual deadline)
> > > as a maximum. That's exactly how I intend it to work. Yes I realise that
> > > the max latency ends up being longer intermittently on the niced task but
> > > that's -in my opinion- perfectly fine as a compromise to ensure the nice
> > > 0 one always gets low latency.
> > I think, it should be possible to spread this max expiration latency across
> > the rotation, should it not?
>
> There is a way that I toyed with of creating maps of slots to use for each 
> different priority, but it broke the O(1) nature of the virtual deadline 
> management. Minimising algorithmic complexity seemed more important to 
> maintain than getting slightly better latency spreads for niced tasks. It 
> also appeared to be less cache friendly in design. I could certainly try and 
> implement it but how much importance are we to place on latency of niced 
> tasks? Are you aware of any usage scenario where latency sensitive tasks are 
> ever significantly niced in the real world?
It depends on how you reconcile "completely fair" and "order of 
magnitude blips in latency." It looks (from the results, not the code) 
as if nice is implemented by round-robin scheduling followed by once in 
a while just not giving the CPU to the nice task for a while. Given the 
smooth nature of the performance otherwise, it's more obvious than if 
you weren't doing such a good job most of the time.

Ugly stands out more on something beautiful!

--
Bill Davidsen <davidsen@xxxxxxx>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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