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From: jlg@lanl.ARPA
Newsgroups: net.arch
Subject: Virtual mem
Message-ID: <34548@lanl.ARPA>
Date: Mon, 9-Dec-85 20:17:53 EST
Article-I.D.: lanl.34548
Posted: Mon Dec  9 20:17:53 1985
Date-Received: Wed, 11-Dec-85 04:09:41 EST
Distribution: net
Organization: Los Alamos National Laboratory
Lines: 27


>   Now people are claiming that 2 Gigabytes means you don't need virtual
> memory.  I'd like to point out that while it probably would mean that your
> program can be completely in main memory, you (at least I) would still want
> the wonders of segmentation (read-only, shared, bounds checks).  Moreover,
> it allows you to not page the entire program into memory from disk.  (Which
> could be expensive and needless for my 1 Gigabyte program (:-)).

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-)?

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 ).

To be sure, there are times when paging would be convenient.  But speed
and convenience are not necessarily compatible.

J. Giles
Los Alamos