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From: bzs@bu-cs.BU.EDU (Barry Shein)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <1177@bu-cs.bu-cs.BU.EDU>
Date: Sat, 6-Sep-86 18:03:48 EDT
Article-I.D.: bu-cs.1177
Posted: Sat Sep  6 18:03:48 1986
Date-Received: Sat, 6-Sep-86 22:14:09 EDT
Organization: Boston U. Comp. Sci.
Lines: 27


From: jlg@lanl.ARPA (Jim Giles)
>The task of zeroing memory on the Cray 2 with 256MW (=2GB) should take
>less than 5-10 seconds.  You must remember that a one MIP machine is
>pathetically slow compared to a Cray (at least for vector operations
>like zeroing memory, searching, sorting, and many scientific
>applications).

That's exactly my point, the Cray "deserves" 2GB of memory (was this a
counter-example? I just said if machine can't zero memory in N seconds
for some small N it probably won't use the memory either, your point
is tautological to that.)

Working backwards, if the Cray can zero 2GB in 10 seconds we get
(assuming, as before, 3 instructions (I) per zero per word, 8 bytes
per word for the Cray) 2GB/8B -> 256M * 3I -> 768MI/10s -> 76.8MIPS,
using your 5 second figure, around 150MIPS.

BUT -- How is that speed being accomplished (and I suspect from the
math above that the Cray is a little faster, no matter)? By
parallelism, vector processors, very non-standard expensive stuff (tho
of course becoming more popular BECAUSE OF THE ABOVE DISCUSSED
PROBLEMS, among others.) The anti-intuitive argument that comes up is
to show that increasing memory size for conventional processors into
the GB range is a losing proposition.

	-Barry Shein, Boston University