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SubjectRe: Ingo Molnar and Con Kolivas 2.6 scheduler patches
Date
On Wednesday 06 August 2003 22:28, Rob Landley wrote:
> 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.)

The upper bound for softrr realtime scheduling isn't based on number of tasks,
it's a global slice of cpu time: so long as the sum of running times of all
softrr tasks in the system lies below limit, softrr tasks will be scheduled
as SCHED_RR, otherwise they will be SCHED_NORMAL.

> In theory, the real benefit of SCHED_SOFTRR is that an attempt to switch to
> it can fail with -EMYBRAINISMELTING up front, so you know when it won't
> work at the start, rather than having it glitch halfway through the run.

Not as implemented. Anyway, from the user's point of view, that would be an
unpleasant way for a sound player to fail. What we want is something more
like a little red light that comes on (in the form of error statistics, say)
any time a softrr process gets demoted. Granted, there may be situations
where what you want is the right behavior, but it's (as you say) a separate
issue of resource allocation.

Regards,

Daniel

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