Subject: Re: [ANNOUNCE] Linux 2.6 Real Time Kernel
From: Lee Revell <rlrevell@joe-job.com>
To: stefan.eletzhofer@eletztrick.de
Cc: linux-kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <20041009213817.GB25441@tier.local>
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	 <1097346628.1428.11.camel@krustophenia.net>
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Date: Sat, 09 Oct 2004 15:47:02 -0400
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On Sat, 2004-10-09 at 17:38, stefan.eletzhofer@eletztrick.de wrote:
> On Sat, Oct 09, 2004 at 03:30:27PM -0400, Lee Revell wrote:
> > On Sat, 2004-10-09 at 17:26, stefan.eletzhofer@eletztrick.de wrote:
> > > On Sat, Oct 09, 2004 at 02:30:28PM -0400, Lee Revell wrote:
> > > > On Sat, 2004-10-09 at 13:41, Karim Yaghmour wrote:
> > > > > Sven-Thorsten Dietrich wrote:
> > > > > >     - Voluntary Preemption by Ingo Molnar
> > > > > >     - IRQ thread patches by Scott Wood and Ingo Molnar
> > > > > >     - BKL mutex patch by Ingo Molnar (with MV extensions)
> > > > > >     - PMutex from Germany's Universitaet der Bundeswehr, Munich
> > > > > >     - MontaVista mutex abstraction layer replacing spinlocks with mutexes
> > > > > 
> > > > > To the best of my understanding, this still doesn't provide deterministic
> > > > > hard-real-time performance in Linux.
> > > > 
> > > > Using only the VP+IRQ thread patch, I ran my RT app for 11 million
> > > > cycles yesterday, with a maximum delay of 190 usecs.  How would this not
> > > > satisfy a 200 usec hard RT constraint?
> > > 
> > > I think the keyword here is "deterministic", isn't it?
> > 
> > Well, depends what you mean by deterministic.  Some RT apps only require
> > an upper bound on response time.  This is a form of determinism.
> 
> Yes. But can you give that upper bound "a priori", that is w/o doing
> measurements with your application?
> 

Yes.  The upper bound on the response time of an RT task is a function
of the longest non-preemptible code path in the kernel.  Currently this
is the processing of a single packet by netif_receive_skb.

AIUI hard realtime is about bounded response times.  How does this not
qualify?

Lee

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