Messages in this thread | | | Subject | Re: [RFC][PATCH 09/22] sched: add period support for -deadline tasks | From | Raistlin <> | Date | Fri, 12 Nov 2010 14:33:47 +0100 |
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On Thu, 2010-11-11 at 20:43 +0100, Peter Zijlstra wrote: > > Since you spotted it... The biggest issue here is admission control > > test. Right now this is done against task's bandwidth, i.e., > > sum_i(runtime_i/period_i)<=threshold, but it is unfortunately wrong... > > Or at least very, very loose, to the point of being almost useless! :-( > > Right, I have some recollection on that. > :-)
> So sufficient (but not necessary) means its still a pessimistic approach > but better than the one currently employed, or does it mean its > optimistic and allows for unschedulable sets to be allowed in? > Tommaso already gave the best possible explanation of this! :-P
So, trying to recap: - using runtime/min(deadline,period) _does_ guarantee schedulability, but also rejects schedulable situations in UP/partitioning. Quite sure it _does_not_ guarantee schedulability in SMP/global, but *should* enable bounded tardiness; - using runtime/period _does_not_ guarantee schedulability nor in UP/partitioning neither in SMP/global, but *should* enable bounded tardiness for _both_.
The *should*-s come from the fact that I feel like I read it somewhere, but right now I can't find the paper(s), not even following the references indicated by Bjorn and Jim in previous e-mails and threads (i.e., I can't find anything _explicitly_ considering deadline!=period, but it might be my fault)... :-(
Thus, all this being said, what do you want me to do? :-D
Since we care about bounded tardiness more than 100%-guaranteed schedulability (which, BTW, neither min{} could give us, at least for SMPs), should we stay with runtime/period? Tommaso, Luca, do you think it would be so bad?
Thanks and Regards, Dario
-- <<This happens because I choose it to happen!>> (Raistlin Majere) ---------------------------------------------------------------------- Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa (Italy)
http://blog.linux.it/raistlin / raistlin@ekiga.net / dario.faggioli@jabber.org [unhandled content-type:application/pgp-signature] | |