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From: mangoe@umcp-cs.UUCP (Charley Wingate)
Newsgroups: net.arch
Subject: Re: Virtual mem
Message-ID: <2511@umcp-cs.UUCP>
Date: Tue, 10-Dec-85 00:36:54 EST
Article-I.D.: umcp-cs.2511
Posted: Tue Dec 10 00:36:54 1985
Date-Received: Wed, 11-Dec-85 04:19:30 EST
References: <34548@lanl.ARPA>
Distribution: net
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 29

In article <34548@lanl.ARPA> jlg@lanl.ARPA writes:

>I know about all the advantages of bounds checks etc.  But on big machines
>I've been without them for so long that I hardly miss them except during
>code development.  A production code runs correctly doesn't it 8-)?

This is fine in systems where there isn't malicious code.  In commercial
applications, one must assume there is deliberately destructive code.

>However, I can't see your point about not loading your whole code.  Why 
>does a program reference any data that it's not going to use?  And if
>it uses the data, then certainly it should be loaded.  If your program
>is a gigabyte long then sooner or later all of that gigabyte must be
>active.  My experience with large codes is that they use nearly all the
>data that they reference nearly all the time (for example, a lattice gauge
>theory code that steps through the entire lattice on the inner loop of
>the code - unless you want to page fault right and left the simplest 
>approach is to load the whole lattice ).

There are lots of programs, however, which do not consist of a huge table
with a little bit of code hanging off the side.  Consider an event-driven
simulation program.  A significant fragment of the code is going to be
simply looking for the next event.  A relatively small area of the data is
going to be exercised disproportionately.  THis even more true of data
retrieval systems.  These programs tend to have data swapping built into
them (since they tend to allow for unlimited expansion through the use of
files), so why should there be an objection to using paging?

Charley Wingate