Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ames!ncar!gatech!hubcap!bjornl
From: bjornl@tds.kth.se (Bj|rn Lisper)
Newsgroups: comp.parallel
Subject: Re: Hmm!
Message-ID: <3405@hubcap.UUCP>
Date: 1 Nov 88 17:58:18 GMT
Sender: fpst@hubcap.UUCP
Lines: 34
Approved: parallel@hubcap.clemson.edu


In article <3363@hubcap.UUCP> vrdxhq!ogccse.ogc.edu!pase@uunet.UU.NET
(Douglas M. Pase) writes:
%mschedlb@hawk.ulowell.edu (Martin Schedlbauer) writes:
%>>  Is the Connection Machiene by ThinkingMachines Inc. a hypercube?
%>No it isn't. A hypercube is an architecture, in which nodes are connected to
%>their Gray-Code neighbours.

%I'm not sure this is correct.  As I recall, there are two networks available
%for use in the CM.  The first is a NEWS network (either a torus or a mesh, I
%don't remember which), and the second is indeed a hypercube.  

First, it is not exactly true that the CM is a hypercube. The CM has its
processors arranged in clusters of 16 each. There is a full crossbar between
the processors in each cluster. The *clusters* are connected to each other
in a hypercube configuration.

I think the NEWS network is really embedded within the hypercube (and the
local cluster crossbars) using binary-reflected Gray encoding. Thus it
doesn't have any physical connections of its own.

%One big difference is that the CM hypercube is synchronous (i.e., a 12-D
%network requires 12 steps to transfer data, even if the target is a nearest
%neighbor).

I wonder if this is true. If there is congestion, messages will have to wait
until the line is free. Therefore it might take more than twelve cycles to
complete an instruction. This implies that there must be some mechanism by
which completion of all transfers is detected. This mechanism should be
able to work even when the transfers complete in less than 12 cycles, i.e.
when all processors issue nearest neighbor transfers.

Any TMC people out there who know the facts?

Bjorn Lisper