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From: ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University)
Newsgroups: comp.sys.mac.system
Subject: Preemption good, time-slicing bad (was Re: All about sys 7.0 <long>)
Message-ID: <1991Apr1.221924.3287@waikato.ac.nz>
Date: 1 Apr 91 10:19:24 GMT
References: <1991Mar26.174540.8425@xn.ll.mit.edu> <1991Mar27.163059.27061@fwi.uva.nl> <7618@idunno.Princeton.EDU>
Organization: University of Waikato, Hamilton, New Zealand
Lines: 37

In article <7618@idunno.Princeton.EDU>, bskendig@set.Princeton.EDU
(Brian Kendig) says:

"Do you want to open up MacWrite II, SuperPaint, HyperCard, and a few
other applications, then have your work in Microsoft Excel be slowed
to a crawl as your Macintosh takes time out every few milliseconds to
make sure MacWrite II hasn't suddenly developed a need to do something?
Preemptive multitasking means that the more applications you have open,
the slower your machine will go..."

I'd like to clarify a couple of things here:

* "Preemptive" multitasking means that, if a task becomes ready to
  run, and the task that's already running is _lower_ in priority, then
  the former can immediately grab the CPU away from the latter.
* If the OS goes one step further, so that a running task is periodically
  forced to give up the CPU to another waiting task of the _same_
  priority, then this is called "time-slicing".

I think preemptive multitasking is a good idea, in both single-user
and multi-user systems. It makes many things simpler (and improves
responsiveness) for both programmers and users.

Multi-user OSes use time-slicing as one technique to ensure that no
user can hog the whole system at the expense of others. In a single-
user system, where you have multiple tasks cooperating, rather than
competing, with each other, I think time-slicing is a poor idea.

Brian Kendig seems to believe that preemptive multitasking systems
also have to do time-slicing. I'd like to make it clear that this is
not the case.

Lawrence D'Oliveiro                       fone: +64-71-562-889
Computer Services Dept                     fax: +64-71-384-066
University of Waikato            electric mail: ldo@waikato.ac.nz
Hamilton, New Zealand    37^ 47' 26" S, 175^ 19' 7" E, GMT+12:00
DOS 2.0: still the biggest jump in functionality in DOS's history.