Date: Wed, 5 Jun 2002 08:51:27 -0500 (CDT)
From: Oliver Xymoron <oxymoron@waste.org>
To: Daniel Phillips <phillips@bonn-fries.net>
cc: linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [ANNOUNCE] Adeos nanokernel for Linux kernel
In-Reply-To: <E17FQzQ-0001T2-00@starship>
Message-ID: <Pine.LNX.4.44.0206050800140.2614-100000@waste.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org

On Wed, 5 Jun 2002, Daniel Phillips wrote:

> On Wednesday 05 June 2002 04:40, Oliver Xymoron wrote:
> > On Wed, 5 Jun 2002, Daniel Phillips wrote:
> >
> > > On Tuesday 04 June 2002 21:29, Oliver Xymoron wrote:
> > > > On Mon, 3 Jun 2002, Daniel Phillips wrote:
> > > >
> > > > > traditional IT.  Not to mention that I can look forward to a sound
> > > > > system where I can be *sure* my mp3s won't skip.
> > > >
> > > > Not unless you're loading your entire MP3 into memory, mlocking it down,
> > > > and handing it off to a hard RT process. And then your control of the
> > > > playback of said song through a non-RT GUI could be arbitrarily coarse,
> > > > depending on load.
> > >
> > > Thanks for biting :-)
> > >
> > > First, these days it's no big deal to load an entire mp3 into memory.
> > >
> > > Second, and of more interest to broadcasting industry professionals and the
> > > like, it's possible to write a real-time filesystem that bypasses all the
> > > normal non-realtime facilities of the operating system, and where the latency
> > > of every operation is bounded according to the amount of data transferred.
> > > Such a filesystem could use its own dedicated disk, or, more practically, the
> > > RTOS (or realtime subsystem) could operate the disk's block queue.
> > >
> > > If I recall correctly, XFS makes an attempt to provide such realtime
> > > guarantees, or at least the Solaris version does.  However, the operating
> > > system must be able to provide true realtime guarantees in order for the
> > > filesystem to provide them, and I doubt that the combination of XFS and
> > > Solaris can do that.
> >
> > Nope, it can't.
> >
> > Just bear in mind that it's next to impossible to avoid throwing the baby
> > out with the bathwater here. Ok, so you've got an RT kernel playing your
> > MP3 alongside your UNIX system - how do you control it? How do you switch
> > tracks? All the latency that you were struggling with in the player is
> > still there in the user interface.

> In the context of an mp3 playback system, worrying about whether the
> controls are realtime seems a little excessive.

Perhaps you've never been a DJ..

> But it's not too hard to do.  Just allow the RTOS to take control of the
> input devices, or at least to insert itself ahead of the general purpose
> operating system in the interrupt pipeline.
>
> As for a GUI with realtime response characteristics, that's more
> challenging, but it has been done.  It's also less important I'd say.  You
> can always fall back on dedicated hardware for the realtime display.

Of course, but then it's no longer part of a general-purpose operating
system. Baby, bathwater. It's easier to tape a Rio onto the side of your
machine.

> > What you really want for an MP3 player is _not_ hard RT, what you want is
> > very reliable low-latency. Which we can do without throwing away most of
> > UNIX.
>
> I think that depends on whether you are an audiophile or not.  Or a
> broadcaster.  If you're a broadcaster, how many mp3 skips will you tolerate
> a year?

If the number is zero, you can't use the facilities of a non-RT operating
system for anything except (possibly lossily) monitoring the output of
such a system. If you feed an RT system with data or control from a non-RT
system, it becomes a non-RT system. If you try to losslessly queue output
from an RT system to a non-RT system, it becomes a non-RT system.

> And who said anything about throwing away most of Unix?

You did, by insisting on absolutes. Hard realtime places many restrictions
on what you can do - it's like having your entire system inside an
interrupt context or a signal handler.

-- 
 "Love the dolphins," she advised him. "Write by W.A.S.T.E.."

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