Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!ukma!psuvm.bitnet!cunyvm!nyser!cmx!billo
From: billo@cmx.npac.syr.edu (Bill O)
Newsgroups: comp.arch
Subject: Re: Is Shared Memory Necessary?
Message-ID: <501@cmx.npac.syr.edu>
Date: 22 May 88 22:05:14 GMT
References: <685@thalia.rice.edu> <43700039@uicsrd.csrd.uiuc.edu>
Reply-To: billo@cmx.npac.syr.edu (Bill O'Farrell)
Organization: Northeast Parallel Architectures Center, Syracuse NY
Lines: 27

In article <43700039@uicsrd.csrd.uiuc.edu> turner@uicsrd.csrd.uiuc.edu writes:

>Why is it that everyone seems to assume that machines must either have
>shared memory OR distributed memory, never a little of both?  From my
>point of view I would like to see a machine with: fast local memory;
>slower, but deeply pipelined, shared memory; and a thin global sync
>bus (for barrier sync).  We have had memory heirarchies for decades
>now, why should they cease to be useful now?

I know of one architecture that has both local memory and globally
shared memory -- the BBN Butterfly.  Is I understand it (correct me if
I'm wrong), the processing nodes on the Butterfly each have memory
which is local in the sense that it can be accessed very quickly by
the local processor, but which is globally shared, in the sense that
all the other processors can access it too, via the big global switch.
Architecturally, each node "thinks" it has a very big globally shared
memory, but software can take advantage of fast local access by
arranging for each processor's most frequently accessed data to be in
that portion of memory which is truly local.

Hmm... I hope I've got that right.

Are there any other multi-processor architectures that have some mix of
both local and shared memory?

Bill O'Farrell, Northeast Parallel Architectures Center at Syracuse University
(billo@cmx.npac.syr.edu)