Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!husc6!mit-eddie!killer!elg
From: elg@killer.UUCP (Eric Green)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Message-ID: <4332@killer.UUCP>
Date: 4 Jun 88 00:53:52 GMT
Article-I.D.: killer.4332
References: <19730@beta.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 38

in article <19730@beta.UUCP>, jlg@beta.UUCP (Jim Giles) says:
> In article <4257@killer.UUCP>, elg@killer.UUCP (Eric Green) writes:
>> in article <5135@nsc.nsc.com>, stevew@nsc.nsc.com (Steve Wilson) says:
>> > In article <19496@beta.UUCP> jlg@beta.UUCP (Jim Giles) writes:
>> >>This problem is an example of the original statement I made on this subject:
>> >>when the ratio of CPU speed to disk speed is high, virtual memory is not
>> >>as attractive.  The problem is not solved by such schemes as having larger
>> 
>> Au contraire, virtual memory is not the villain you make it out to be. 
>> .... it takes little more time to page-fault
>> a program into memory than it takes to load it straight into RAM, especially
>> with today's faster CPU's.
>> 
> Au contraire yourself!  Memory needn't be big enough for the whole program
> and data in order to benefit from eliminating VM.  There are whole classes
> of programs that know AHEAD OF TIME what data they are going to need next.

Clarification: I was not addressing virtual memory vs. no virtual memory. I
was addressing the "problem" of ratio of CPU speed to disk speed.

The stereotypical example of a program that does not run well under VM is the
large scientific array processing program. Such a program marches from one end
of a huge array to the other, and by the time the last pages are accessed, the
first pages have already been paged out of RAM (meaning that the next pass
results in the whole array being paged back into RAM).

Without VM, the program must explicitly handle the segmentation of data, and
can make sure that as much is kept loaded as will fit in memory. With VM, the
problem could possibly be handled -- but hasn't been, in the past. 

None of which has anything to do with the "problem" of "disk access so slow
that it takes longer to load the page than it takes to execute it". VM makes
little difference, there.

--
    Eric Lee Green                     {cuae2,ihnp4}!killer!elg
         Snail Mail P.O. Box 92191 Lafayette, LA 70509              
"Is a dream a lie if it don't come true, or is it something worse?"