Re: Ingo Molnar and Con Kolivas 2.6 scheduler patches

[Helge Hafting <helgehaf@aitel.hist.no>]

Rob Landley wrote:
[...]
> 
> Thinking out loud for a bit, please tell me if I'm wrong about SCHED_SOFTRR...
> 
> Whatever the policy is, there's only so many ticks to go around and there is 
> an overload for which it will fail.  No resource allocation scheme can 
> prevent starvation if there simply isn't enough of the resource to go around.
> 
> So, how does SCHED_SOFTRR fail?  Theoretically there is a minimum timeslice 
> you can hand out, yes?  And an upper bound on scheduling latency.  So 
> logically, there is some maximum number "N" of SCHED_SOFTRR tasks running at 
> once where you wind up round-robining with minimal timeslices and the system 
> is saturated.  At N+1, you fall over.  (And in reality, there are interrupts 
> and kernel threads and other things going on that get kind of cramped 
> somewhere below N.)


I don't know how this particular scheduler fail, but the problem
exists for any real-time system.  Nobody can run "N+1" guaranteed 
low-latency
tasks where N is the max, that is obvious.

A generic os scheduler can run almost any amount of tasks, with latencies
proportional to the amount of tasks.

A good RT scheduler won't even _try_ to run "N+1" RT tasks.  The
last task will either fail to start, or fail the attempt to
increase its priority into RT.  You may then kill (or un-prioritize)
some other RT task and try again.

Helge Hafting