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From: jlg@lanl.ARPA (Jim Giles)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <7965@lanl.ARPA>
Date: Thu, 25-Sep-86 15:29:48 EDT
Article-I.D.: lanl.7965
Posted: Thu Sep 25 15:29:48 1986
Date-Received: Fri, 26-Sep-86 00:59:29 EDT
References: <1130@bu-cs.bu-cs.BU.EDU> <7144@lanl.ARPA> <7148@lanl.ARPA> <609@nike.UUCP> <7839@lanl.ARPA> <627@nike.UUCP>
Reply-To: jlg@a.UUCP (Jim Giles)
Organization: Los Alamos National Laboratory
Lines: 22
Keywords: virtual memory

In article <627@nike.UUCP> lamaster@pioneer.UUCP (Hugh LaMaster) writes:
>One important point that I forgot to mention in previous posting:
>On a machine with memory mapping (Cyber-205 for example) there is only a very
>small penalty for reclaiming a small amount of memory for another task.  On a
>very large main memory system, this is an important point.  A 256 MW Cray-2
>(with approx. 4 ns clock) would take 1 second to completely copy main memory.
>Unfortunately, this is exactly what is required to reclaim memory on the Cray-2
>or any other machine which requires memory to be contiguous.
>...

This is not really true.  There is no reason inherent in a non-VM
system which requires it to swap the whole large program out in order
to make room for the smaller one.  The only absolute requirement is
that the large program must be entirely resident while it is running.

As it happens, the systems currently running on Cray machines drop
entire memory images when swapping for space.  But this is not a
requirement - only a simplification of the duties of the operating
system.

J. Giles
Los Alamos