Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!ll-xn!nike!oliveb!hplabs!sdcrdcf!pmontgom
From: pmontgom@sdcrdcf.UUCP (Peter Montgomery)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <3013@sdcrdcf.UUCP>
Date: Wed, 17-Sep-86 21:29:54 EDT
Article-I.D.: sdcrdcf.3013
Posted: Wed Sep 17 21:29:54 1986
Date-Received: Fri, 19-Sep-86 23:17:28 EDT
References: <1130@bu-cs.bu-cs.BU.EDU> <7144@lanl.ARPA>
Reply-To: pmontgom@sdcrdcf.UUCP (Peter Montgomery)
Organization: System Development Corporation R&D, Santa Monica
Lines: 22
Keywords: VM economy tradeoff


        I once was a system programmer for a CDC 7600 site.  We ran many
scientific programs, but lacked virtual memory.  Some programs used almost
all of memory.  These jobs might run faster than the same ones would with
less memory, but overall system performance suffered.  For example, a
compilation might not fit in memory alongside a huge scientific job.  If
the compilation runs alone, the CPU will be idle much of the time it runs.
Yet the scientific program might have considerable unused code and/or data
(e.g., the input data for this run does not select Calcomp plots, so those
routines are never called and their data areas are never referenced;
another likely possibility is that some dimensions are much larger than
required).  If either job runs alone, then it wants as much memory as it
can get.  With virtual memory, the operating system can simultaneously
load the heavily used parts of BOTH jobs in the machine, for better
overall performance.
-- 
			Peter Montgomery

	{aero,allegra,bmcg,burdvax,hplabs,
	 ihnp4,psivax,randvax,sdcsvax,trwrb}!sdcrdcf!pmontgom

Don't blame me for the crowded freeways - I don't drive.