From: mikeh@starnine.com (Mike Haas)
Subject: Re: CBM mention on 12/11/92 Computer Chronicles
Date: Wed, 13 Jan 1993 05:09:05 GMT
Organization: StarNine Technologies, Inc.

In article <1993Jan7.011228.22981@msi.com> bill@west.msi.com writes:
>Mike Haas (mikeh@starnine.com) wrote:
>: In article <1993Jan4.204108.367@sol.ctr.columbia.edu> jerry@msi.com (Jerry Shekhel) writes:
>
>: The benefit of VM is allowing more apps to run.  Period.  It can only
>: have a negative effect on performance, the effect may be large or
>: not-so-large, but it is inescapble.
>
>This is not necessarily true.  Lets have an example:
>You have a machine with 8MB RAM.  First you run using VM.  The OS has a
>total size of 3MB, a working set of 1.5MB.  The first application is 4MB,
>a working set of 2MB.  The second application is 4MB, a working set of
>3MB.  The total number of RAM needed for both applications to work
>without swapping is 1.5+2+3 = 5.5 MB.  Without VM, the system would need
>is 3+4+4 = 11MB.  You can run them all.  If you did have 11MB then both
>applications would run at the same speed as the VM system with 8MB.
>Applications only experience slowdown a VM multitasking system when there
>isn't enough RAM to hold the application's working set.  If you don't
>understand what I am talking about, take Jerry's advice and take a decent
>OS course.

Magic.  Anytime a VM system goes to disk for either working set or
program code, it will experience a slowdown.  Neither are fetched until
needed.  When they are needed, you not only have to wait for them to
be loaded in, but probably have to write something out to free up some
real physical memory.

>
>: >Seriously, all these claims of VM slowing down performance are absolute
>: >rubbish.  Take a course in OS design.
>: 
>: What you just said is pure rubbish, Jerry.  Ever run Unix in 4 Meg?
>: You'll get bored waiting for 'ls'.
>
>I have run Unix in 512K.  'ls' worked just fine.

Unix kernals are greater than 512K, and can't be swapped.

>
>: Any program that has to periodically swap code segments will suffer
>: performance degredation in it's own execution.  It doesn't have
>: to affect other running programs (unless they get into contention for
>: the I/O system), but it is an inescapable fact that that program
>: will run slower than if it weren't swapped out.
>
>What if a program doesn't have to periodically swap code?  What if it
>only swaps out code after the its initization is over?  Doesn't seem like
>any problems would occur in a VM system.

Then you are not using the VM if not swapping things

>
>: The purpose of VM is to allow more program execution with smaller
>: amounts of memory.  There is a necessary negative effect on system
>: performance.
>
>You are drawing a conclusion that does not follow your facts.  One fact
>you need for your conclusion to be correct: The working sets of all
>programs in the system must exceed all availible RAM.  If all pages of
>all working sets of all applications fit into availible RAM, then after
>startup, there is no further paging, and therefore no more degredation of
>performance.

So basically, you've said that VM doesn't slow things down if you don't
use it.  I'll agree with that.

But when you swap, it will take time away from the application being
swapped, and possible other processes too if there is I/O contention.


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