Xref: utzoo comp.sys.mac:14178 comp.windows.misc:334
Path: utzoo!mnetor!uunet!nuchat!peter
From: peter@nuchat.UUCP (Peter da Silva)
Newsgroups: comp.sys.mac,comp.windows.misc
Subject: Re: Preemptive and Nonpreemptive Multitasking
Message-ID: <826@nuchat.UUCP>
Date: 19 Mar 88 14:04:52 GMT
References: <4129@hoptoad.uucp> <283@rhesus.primate.wisc.edu> <1710@ssc-vax.UUCP> <7720@apple.Apple.Com>
Organization: Public Access - Houston, Tx
Lines: 70
Keywords: window human computer interface

In article <7720@apple.Apple.Com>, dwb@Apple.COM (David W. Berry) writes:
> 	A quick summary of pro's and con's of preemptive and nonpreemptive
> multitasking:

	A few comments based on the fact that you're comparing
yourself to a non real-time operating system. For single-user
use, a real-time operating system (like Amiga's Exec) addresses
virtually every complaint you have with concurrency.

> Task switches:
> 	With non-preemptive multitasking task switches are simpler
> because you know the exact state of the machine when the switch
> occurs.  With preemptive you're never quite sure.

On a machine without memory protection, task switches can be as simple
as:

	timer interrupt occurs in the context of the old task.
	push all registers on old task's stack.
	pop all registers off new task's stack.
	return in the context of the new task.

> Indivisible operations:
> 	Nonpreemptive multitasking makes indivisible operations a snap
> because the program is in control of when it let's go of the processor.
> With preemptive multitasking you're never sure when the processor will
> go away.

So you use semaphores or formal messages to synchronise things. On UNIX
this is handled invisibly by the kernel: a task switch occurs under
control of the operating system. On the Amiga, which has a much more complex
shared memory environment, you use semaphores or use standard library
calls that do the semaphoring for you. I don't care that DrawImage does
WaitBlits and OwnBlits and LockLayers and stuff.

> Inopportune task switches
> 	Preemptive multitasking allows task switches at inopportune
> times.  Ever have your windowing system lose the processor while your
> dragging something around with the mouse, makes for real jerky mouse
> movement.

Nope. Never had any problew3ms with that. The mouse motion is under control
of a high-priority realtime task called "Intuition".

> Program control
> 	Nonpreemptive multitasking gives the program much more precise
> control of when it get's switched.

Not at all. I can keep from getting switched any time I need to:

	Forbid();
	Do something that needs to be atomic.
	Permit();

> Fairness
> 	Preemptive multitasking is in general "fairer" in that all
> tasks have a more or less equal chance of getting processor time.
> This can be modified by priorities, but no one process is going to
> get all the processor.  This is desireable in a multiuser timesharing
> environment.  It's a much less clear win in a singleuser environment
> since it means that tasks that I'm not clearly interested will be
> getting a fair share of the processor.

If the tasks are I/O bound they'll be spending most of their time on
the wait queue. If they're CPU bound you can always knock their priority down
a peg.
-- 
-- a clone of Peter (have you hugged your wolf today) da Silva  `-_-'
-- normally  ...!hoptoad!academ!uhnix1!sugar!peter                U
-- Disclaimer: These aren't mere opinions... these are *values*.