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SubjectRe: Ingo Molnar and Con Kolivas 2.6 scheduler patches
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
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

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