Messages in this thread | | | Date | Fri, 12 Nov 2010 18:54:35 +0100 | From | Luca Abeni <> | Subject | Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline tasks |
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On 12/11/10 18:51, Peter Zijlstra wrote: > On Fri, 2010-11-12 at 18:41 +0100, Luca Abeni wrote: >>> 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. >> [...] >> BTW, sorry for the shameless plug, but even with the current >> SCHED_DEADLINE you are not forced to dimension the runtime using the >> WCET. > > Yes you are, it pushes the deadline back on overrun. I think in case of overrun it postpones the deadline (by a period P), avoiding to execute the task until the end of the current period, right?
> The idea it to > maintain the deadline despite overrunning your budget (up to a point). > > The paper we're all talking about is: > > A. Mills and J. Anderson, " A Stochastic Framework for Multiprocessor > Soft Real-Time Scheduling", Proceedings of the 16th IEEE Real-Time and > Embedded Technology and Applications Symposium, pp. 311-320, April 2010. > http://www.cs.unc.edu/~anderson/papers/rtas10brevised.pdf I see... This is a different approach to stochastic analysis, which requires modifications to the scheduler.
In the analysis I mentioned, you still enforce a maximum runtime C every period P, but C can be smaller than the WCET of the task. If C is larger than the average execution time, you can use queuing theory to find the probability to miss a deadline (or, the probability to finish a job in a time x * P).
Luca
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