Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline tasks

[Luca Abeni]

Hi James,

On 15/11/10 19:37, James H. Anderson 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.
> > I know, and it's very reasonable. The point I'm trying to make is that
> > resource reservation tries to address the very same issue.
> > I am all but against this model, just want to be sure it's not too much
> > in conflict to the other features we have, especially with resource
> > reservation. Especially considering that --if I got the whole thing
> > about this scheduler right-- resource reservation is something we really
> > want, and I think UNC people would agree here, since I heard Bjorn
> > stating this very clear both in Dresden and in Dublin. :-)
> >
> > BTW, I'm adding them to the Cc, seems fair, and more useful than all
> > this speculation! :-P
> >
> > Bjorn, Jim, sorry for bothering. If you're interested, this is the very
> > beginning of the whole thread:
> > http://lkml.org/lkml/2010/10/29/67
[...]
> If you're talking about our most recent "stochastic" paper, it is about
> supporting
> soft real-time task systems on a multiprocessor where resource
> reservations are
> used. The main result of the paper is that if you provision the
> reservation for a
> task slightly higher than it's average-case execution time, and if you
> use a
> scheduling algorithm (like global EDF) that ensures bounded tardiness
> (w.r.t.
> these reservations), then the task's expected tardiness will be bounded
> and the
> expected bound does not depend on worst-case execution times. I'm not
> sure if
> slack-reallocation methods have come up in this discussion (sorry, I'm
> really
> pressed for time and didn't look), but we didn't get into that in our
> paper.
So, if I understand well (sorry, I am just trying to make a short 
summary to check if we are aligned) your analysis is similar to the one 
presented in the papers I mentioned earlier in this thread (different 
stochastic modelling, but similar approach): you analyse a reservation 
in isolation and you provide some stochastic tardiness guarantees based 
on an (e_i, p_i) service model.... Right?

If my understanding is correct (please, correct me if I am wrong), your 
analysis can be applied even with the current version of Dario's patch 
(I mean: no modifications to the patch are needed for removing 
assumptions about WCET knowledge... Your paper uses a sporadic server 
for the reservation mechanism, but I think a CBS can work too...).


			Thanks,
				Luca
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