Messages in this thread | | | Subject | Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline tasks | From | Peter Zijlstra <> | Date | Fri, 12 Nov 2010 17:04:01 +0100 |
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On Fri, 2010-11-12 at 16:36 +0100, Raistlin wrote: > But at this point I can't avoid asking. That model aims at _pure_ > hard real-time scheduling *without* resource reservation capabilities, > provided it deals with temporal overruns by means of a probabilistic > analysis, right?
From what I understood from it, its a soft real-time scheduling algorithm with resource reservation. It explicitly allows for deadline misses, but requires the tardiness of those misses to be bounded, ie. the UNC soft real-time definition.
The problem the stochastic execution time model tries to address is the WCET computation mess, WCET computation is hard and often overly pessimistic, resulting in under-utilized systems.
By using the average CET (much more easily obtained) we get a much higher system utilization, but since its an average we need to deal with deadline overruns due to temporal overload scenarios.
Their reasoning goes that since its an average, an overrun must be compensated by a short run in the near future. The variance parameter provides a measure of 'near'. Once we've 'consumed' this short run and are back to the average case our tardiness is back to 0 as well (considering an otherwise tight scheduler, say P-EDF), since then we've met the bandwidth requirements placed by this scheduler.
And since the pure statistics allow for an arbitrary large deviation from the average it also requires a max runtime in order to be able to place a bound on tardiness.
So for G-EDF with stochastic ET we still get a bounded tardiness, its a simple sum of bounds, one due to the G in G-EDF and one due to the stochastic ET.
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