>>>>> "David" == David Schleef <[EMAIL PROTECTED]> writes:
David> On Mon, Sep 27, 1999 at 04:23:45PM +0200, David Olofson wrote:
>> [EMAIL PROTECTED] wrote: > > Depends on what you want to do. I
>> think RT/Beowulf is a natural combination > for systems where, for
>> example, one needs a supercomputer level of > visualization or
>> simulation connected to RT control.
>>
>> And while we're on the Beowulf subject; what's the latest news in
>> the network drivers/RTLinux area? I have seen some interesting
>> documents on RETHER, but I'd like to see some more figures on
>> actual transfer rates and latencies that can be achieved with
>> RTLinux drivers.
David> IMO, Rether is a good implementation of a poor idea. At
David> least, it is a poor idea in the RTLinux context. It is a good
David> idea in the Rether context. Keep in mind, also, that Rether
David> is designed for "guaranteed bandwidth", not "real-time." In
David> the soft-real-time world, they are much the same.
David> Rether, for those who do not know, builds a token-passing
David> layer between the ethernet hardware and then uses token
David> passing to moderate each host's use of the ethernet. It is
David> much like a parliament, where each speaker must have the
David> "floor" -- each host must have the "wire".
David>...
David> For hard-real-time networking, peer-to-peer token passing is a
David> bad idea, since it imposes a significant overhead at real-time
David> priority for all hosts for token maintenence -- increased
David> complexity when trying to design systems. Even worse, failure
David> of any host means failure of the network, since correct token
David> passing relies on all hosts. Recovery of the Rether token
David> appears to be one of the major difficulties that the Rether
David> research group solved.
It's a well known problem with a well known solution, first solved in
IEEE 802.4 and copied in FDDI.
On the other hand, there's a good reason why 802.4 is dead. Doing the
same thing only more slowly in software seems like an eminently bad
idea. Or, as a former boss put it so nicely, "reinventing the flat
tire".
paul
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For more information on Real-Time Linux see:
http://www.rtlinux.org/~rtlinux/
