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From: drew@everexn.com (Drew Rogge)
Newsgroups: comp.sys.mac.system
Subject: Re: True Multitasking
Message-ID: <1991Jan18.195343.6636@everexn.com>
Date: 18 Jan 91 19:53:43 GMT
References: <1991Jan15.233452.1163@maverick.ksu.ksu.edu> <42602@ut-emx.uucp> <11745@goofy.Apple.COM>
Organization: Everex Systems, Sebastopol
Lines: 30

In article <11745@goofy.Apple.COM> lsr@Apple.com (Larry Rosenstein) writes:
>MPW tools (including the compiler) run nicely in the background.  In fact,
>it is very easy to take some generic C utility and make it run in the 
>background under MPW.

Does anyone know if there is a function that an MPW tool can call which will
yield control of the CPU for a while to give other tasks a chance to run?
I ported Dave Buck's raytracer, DKBTrace, to run as an MPW tool but the
response time of the system was horrible when it was running. The program
is very compute bound and doesn't call out to the system very often.

On the issue of preemptive vs. cooperative multitasking, it seems to me
that only the OS really knows what's going on in the entire system. I
imagine that there's a lot of time wasted by tasks interrupting their
operation just to give the operating system a chance to check if there's
anything else to do.

One thing to remember is that preemptive multitasking does not necessarily
mean that every task gets allotted equal slices of time in a round-robin
fashion. There are many methods for tasks which don't have anything to do
to cause themselves to be temporarily removed from the scheduling queue.
It is also possible for "foreground" task(s) to be allocated a larger
percentage of the system resources.

I could be wrong, but I think that one problem with implementing preemptive
multitasking on a Mac are sections of "critical" code in applications
which occur when the low memory globals are being accessed or modified.

Drew Rogge
drew@everexn.COM