From: bill@snoopy (Bill Poitras)
Subject: Re: CBM mention on 12/11/92 Computer Chronicles
Reply-To: bill@west.msi.com
Organization: Molecular Simulations, Inc.
X-Newsreader: Tin 1.1 PL5
Date: Thu, 7 Jan 1993 01:12:28 GMT

Mike Haas (mikeh@starnine.com) wrote:
: In article <1993Jan4.204108.367@sol.ctr.columbia.edu> jerry@msi.com (Jerry Shekhel) writes:
: >Michael van Elst (mlelstv@speckled.mpifr-bonn.mpg.de) wrote:
: Oh, so VM only introduces INsignificant performance degredation, eh?
: Really, Jerry, you need to take a course in real-time OS applications.

I don't think the discussion is completely limited to real-time OS's.
Many applications are not real-time.  And for the ones that are, some
OSes provide a scheduling priority for them.  OS/2 2.0 is one of them.

: 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.

: >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.

: 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.

: 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.

--
+-----------------+-------------------------------+-------------------------+
| Bill Poitras    | Molecular Simulations Inc.    | Tel (408)522-9229       |
| (bill)          | Sunnyvale, CA USA             |                         | 
|                 | FAX (408)732-0831             | bill@west.msi.com       |
+-----------------+-------------------------------+-------------------------+

Newsgroups: comp.sys.amiga.advocacy