Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ames!hc!beta!jlg
From: jlg@beta.UUCP (Jim Giles)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Message-ID: <20014@beta.UUCP>
Date: 6 Jun 88 23:16:07 GMT
References: <19730@beta.UUCP> <4332@killer.UUCP> <5144@nsc.nsc.com>
Organization: Los Alamos Natl Lab, Los Alamos, N.M.
Lines: 30

In article <5144@nsc.nsc.com>, stevew@nsc.nsc.com (Steve Wilson) writes:
> If a "typical" scientific application requires 64Mb of array space to
> fit the entire data set of the problem into physical memory AND
> such memory is present, then the correct solution is to implement the
> VM hardware in such a way to allow all 64Mb to be mapped at the same
> time.  Thus, paging is prevented in the "typical" case.

64MB is only 8 million words - about typical for a small scientific
production code.  Most new supercomputers have more memory than that.
Even so, most scientific production codes will soon be scaled up so that
even the 32MW (that's million words) machines aren't big enough.  I've
heard that the Cray 2's with 256MW are only 'just' big enough for the
applications that various users have (presumably they've scaled their
problems down a bit in order to fit them in memory).  The 'typical'
scientific code is one that just barely fits the current maximum
archetecture when the program is run in its simplest configuration.  In
other words, no matter how much memory you give me, I can find people who
already have legitimate codes that use ALL of it.  Not only that - these
people would soon be asking for more.  The one other thing these people
have in common is that they neither need, nor particularly want, VM on
any new machine.

J. Giles
Los Alamos

P.S.  This discussion comes up about once every 6 months.  I should point
      out that I have no particular objection to VM in most computing
      environments.  I wouldn't accept a machine like a SUN workstation
      if it didn't have VM.  I probably wouldn't care much for a CRAY
      that DID have VM.  The claim that VM is UNIVERSALLY desirable
      seems to me to be unreasoned dogma.