Re: [linux-audio-dev] Fwd: [rtl] Beowulf and RTLinux (and NIC latency)

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Subject: Re: [linux-audio-dev] Fwd: [rtl] Beowulf and RTLinux (and NIC latency)
From: David Olofson (audiality_AT_swipnet.se)
Date: ke syys   29 1999 - 19:16:42 EDT


On Thu, 30 Sep 1999, Paul Barton-Davis wrote:
> >Besides, why would you drop communication for 10 ms in a system that
> >constantly transfers data with a cycle time of less than 10 ms?
>
> 10ms was just a guess.

Yeah, and it doesn't matter what the figure is; it's the worst case that
matters, and it must be significantly less than 1 ms for really cool real time
audio stuff.

> Even so, because the nodes are going to be loosely coupled, you're not
> going to be communicating all the time. It may be that the processing
> net on one node is idle for a while (you turn off the TrueVerb, for
> example). You'll need to be ready for the "restart cost", whatever it
> is.

Both there problems are due to system design errors, not hardware problems.

1) You don't do low latency, high bandwidth streaming over a loosely
   coupled SAN. There will be two cards and one cable for every single
   RT connection path. (Well, this is supposed to be an alternative to
   $$,$$$ SMP boxes - what did you expect?)

2) You never *ever* temporarily turn parts of a running hard real time
   system off. If you do, you'll just have to face the brutal truth;
   you may have an unbounded restart time until your system is a real
   time system again. (Solution: Initialize plug-ins first in a low
   priority thread - then plug them into the engine.)

> I'll try to check into what I'm half-remembering about this. I don't
> know where the delay can come from - I should try to find the guy I
> knew who worked on this stuff (which was an early precursor to
> Beowulf).

If it's loosely couple networks with switches or similar solutions, I'm not
suprised that there are latency problems. I'm pretty sure the problem is the
actual reconnecting of nodes after being silent for a while.

//David

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