Xref: utzoo comp.sys.mac:14119 comp.windows.misc:320
Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!claris!apple!goldman
From: goldman@Apple.COM (Phil Goldman)
Newsgroups: comp.sys.mac,comp.windows.misc
Subject: Re: Shy pre-emption++
Message-ID: <7731@apple.Apple.Com>
Date: 18 Mar 88 02:37:07 GMT
References: <7670@apple.Apple.Com> <6970@drutx.ATT.COM> <2154@polya.STANFORD.EDU>
Reply-To: goldman@apple.UUCP (Phil Goldman)
Organization: Apple Computer Inc, Cupertino, CA
Lines: 30

In article <2154@polya.STANFORD.EDU> ali@polya.UUCP (Ali Ozer) writes:
>I'd hate to run any sort of interactive program on such a system, especially
>if anyone is ray-tracing! 8-) But really, there are some other features that
>Unix (and other multitasking systems provide) that you really do need.
>Task priorities, for instance. Consider the Amiga: You run a ray tracer
>and a word processor. Set the ray tracer's priority to a low value, and leave
>the WP at the default. You are assured of getting immediate response as soon
>as you hit any key in your WP, while the ray tracer (which we assume
>is a CPU hog) will soak up any unused cycles from right between your fingers.
>But you won't even know the ray tracer is running.  
>

I think you have a point, but for a different reason.  The only
time you want the WP to have highest priority is when you  are interacting
with it.  At all other times you want it to have the lowest priority.
MultiFinder accomplishes this by guaranteeing highest priority to the
application with the frontmost layer, at the expense of all other
layers.  However, when the WP is not frontmost, it gets no time at all
and will never interrupt the ray tracer from its task.

The only nicety in your example above is that *as soon as* you hit the key
you get switched to the WP, but this is made possible because of preemption,
not task priorities.  Without preemption all the prioritization can't get the
CPU to the WP's event handling code any quicker than MultiFinder does.

One more point about prioritization is that it had better be dynamic
(e.g. frontmost task vs. the rest in MF).  Otherwise, priorities assigned
in a vacuum will not work well with different task mixes.

-Phil Goldman
Apple Computer