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

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Date:	Fri, 12 Nov 2010 19:56:55 +0100
From:	Raistlin <raistlin@...ux.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>, oleg@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Claudio Scordino <claudio@...dence.eu.com>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>,
	Bjoern Brandenburg <bbb@...il.unc.edu>,
	"James H. Anderson" <anderson@...unc.edu>
Subject: Re: [RFC][PATCH 18/22] sched: add reclaiming logic to -deadline
 tasks

On Fri, 2010-11-12 at 17:04 +0100, Peter Zijlstra wrote:
> 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. 
>
Mmm... I've gone through it (again!) quickly, and you're right, it
mentions soft real-time, and I agree that for those systems average CET
is better than worst CET. However, I'm not sure resource reservation is
there... Not in the paper I have at least, but I may be wrong.

> 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

And these should be from where this specific discussion starts (I hope,
the mirror is not updated yet I guess :-( ):
 http://lkml.org/lkml/2010/10/29/49
 http://groups.google.com/group/linux.kernel/msg/1dadeca435631b60

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@...ga.net /
dario.faggioli@...ber.org

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