[Beowulf] [jak at uiuc.edu: [Xgrid] Re: megaFlopsper Dollar? real world requirements]

[Jim Lux]

----- Original Message -----
From: "Eugen Leitl" <eugen at leitl.org>
To: <Beowulf at beowulf.org>
Sent: Tuesday, May 17, 2005 1:51 AM
Subject: [Beowulf] [jak at uiuc.edu: [Xgrid] Re: megaFlopsper Dollar? real
world requirements]


> Typical 1394
> devices have 2 external ports plus an internal port, interconnected at a
> sort of "hub", but it's not a passive hub. It has significant smarts,
maybe
> a "switch" might be a better conceptual model.

  The term "switch" in the networking world implies something very
  specific, mostly data isolation.  I've often wondered if FireWire
  "hubs" do this?  Given the name "hub", I've often assumed a repeater
  based technology, but I realize that's my networking bias.
-----

You're right.  1394 "hubs" do "edit" the data heading through.  They're not
a passive repeater.  However, they do have a capability for very low latency
pass through (that is, they're not store and forward, at least at a big
scale).

---
> channel available for "ad-hoc" messages.

  This is another reason I've always assumed FireWire "hubs" do not do
  bandwidth isolation.  If they did, each channel allocation would have
  to be per-link, rather then per-network, and it seems the resource
  allocation system would get quite complex (and that every interface
  would require a large amount of logic to perform such calculations).

----

The resource allocation system IS "per link" and quite complex, particularly
when you consider that it has to support hot-plug and unplug, etc.,
requiring renegotiation on the fly (without interrupting ongoing
transmissions).

 The saving grace is that because it's consumer oriented, there's always the
"cycle the power" option when things get too screwed up.


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