Re: periods and deadlines in SCHED_DEADLINE

From: Bjoern Brandenburg
Date: Wed Aug 04 2010 - 01:02:39 EST

On Aug 3, 2010, at 5:46 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Sun, 2010-07-11 at 08:42 +0200, Bjoern Brandenburg wrote:
>> If you want to do G-EDF with limited and different budgets on each CPU
>> (e.g., G-EDF tasks may only run for 100 out of 1000 ms on CPU 0, but
>> for 400 out of 1000 ms on CPU 1), then you are entering the domain of
>> restricted-supply scheduling, which is significantly more complicated
>> to analyze (see [1,2]).
> Would making the thing homogenious by assuming the worst for all cpus
> make the analysis easier? That is, in the above example, only allow the
> G-EDF scheduler to run for 100 out of 1000 ms on both cpus.

It would, but that severely limits your non-G-EDF tasks. What if you want to provision a P-EDF task with a period of ten milliseconds? If you allow a G-EDF slice of guaranteed 100 ms, then your P-EDF task might miss a deadline if it arrives at the wrong time. If you let it preempt the G-EDF slice, then you get "out of sync" and the analysis can become tricky. If you scale down the G-EDF time slice length to less than ten ms, then overheads increase needlessly on all processors. EDF-HSB addresses specifically this problem (for the case where G-EDF is only used for soft tasks):

B. Brandenburg and J. Anderson, âIntegrating Hard/Soft Real-Time Tasks and Best-Effort Jobs on Multiprocessorsâ, Proceedings of the 19th Euromicro Conference on Real-Time Systems (ECRTS 2007), pp. 61-70. IEEE, July 2007.

- BjÃrn--
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