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From: jlg@lanl.ARPA (Jim Giles)
Newsgroups: net.arch
Subject: Re: Very large memories
Message-ID: <7150@lanl.ARPA>
Date: Fri, 5-Sep-86 18:13:06 EDT
Article-I.D.: lanl.7150
Posted: Fri Sep  5 18:13:06 1986
Date-Received: Fri, 5-Sep-86 21:59:56 EDT
References: <5100120@ccvaxa>
Reply-To: jlg@a.UUCP (Jim Giles)
Organization: Los Alamos National Laboratory
Lines: 26

In article <5100120@ccvaxa> srb@ccvaxa.UUCP writes:
>
>Saying that "you only need as much memory as you can clear in
>a second (or some similar measure)" implicitly assumes lots of
>things:
>....
>    2)	The access pattern frequencies to memory are reasonably near
>	exponential.  I.e., a minority of locations is accessed
>	most frequently, with increasingly smaller frequency of
>	access to an increasingly large fraction of the memory
>	up to a cumulative (100% - epsilon).
>...
>It is not hard to think of real applications where one or more of
>the above implicit assumptions will be violated.  Large
>data bases, tables of pre-computed cryptographic or scientific
>functions, real-time tasks where access cannot be predicted but
>the data is needed in real-time, and main store backed only by
>a slow-seek-time laser disk come to mind.  I'm sure people like
>Hector Garcia-Molina can list some more of these for us.

Most scientific codes I'm familiar with violate this second assumption.
The entire memory requirement of a Hydro code for example is referenced
with equal frequency in a cyclical manner (once per time-step).

J. Giles
Los Alamos