Re[2]: [Am-info] O/S 2 support - slightly off topic (my apologies)

[Eric M. Hopper]

On Sat, 2002-03-16 at 14:12, Mitch Stone wrote:
> Allow me to ask a simple-minded technical question: what if anything is 
> the relationship between multithreading and preemptive multitasking?

Multithreading and preemptive scheduling are, in one sense, strongly
related, and in another, orthogonal concepts.  Most people do not think
of cooperative multithreading as multithreading at all.  So, in most
people's minds, multithreading is something that only happens when you
have preemptive scheduling.

> The main reason I ask, is because for years we heard that the MacOS was 
> technically inferior to Windows because it used a less sophisticated 
> scheme for multitasking (cooperative). This method gave explicit CPU 
> priority to the front-most active application. What I've found since 
> moving to the threaded OS X, is that the performance of some power-hungry 
> apps suffers because these priorities can no longer be set (as readily) by 
> the user.

Actually, here's a good explanation of the terms preemptive,
cooperative, multitasking, and multithreading in the context of managing
CPU resources...

Multitasking vs. multithreading refers to two different ways to divide
things up.  I liken multitasking to having a lot desks (think of the
inside of your computer as a frantic bookkeeping bureaucracy) on them
and specific in and out boxes.  I liken multithreading to having one
desk with lots of people using it simultaneously and having all of them
have some kind of agreement about which one gets to work on which pieces
of paper when so there's no confusion.

To extend the desktop metaphor further...

Cooperative scheduling is like a single guy doing a small, complete task
and then moving on, either to another group of papers on the same desk
(multithreading) or another desk (multitasking).  Preemptive scheduling
is like having lots of guys.  If you only have one CPU, all the guys but
one are halted in time while the one works, but which one is working and
which one is halted in time changes around semi-randomly (according to
the OSes scheduling algorithm).  If you have two CPUs, all but two guys
are halted in time, etc...

Multithreading systems almost invariably (with the exception of some
specialized embedded systems) also do multitasking.  Not all
multitasking systems can do multithreading.

Preemptive multithreading is hard and error prone because preemptive
means you have several actors, and multithreading means a single desk of
papers.  You end up having to have a complex agreement between all the
actors about who gets to modify which piece of paper when in order to
avoid confusion.  That agreement is very difficult to get right.

This is a big issue for me because my big programming project (the
StreamModule system) is something that allows you to do cooperative
multithreading.  Later, it will allow (in a very restricted and
disciplined way) a mix of cooperative and preemptive multithreading.

I hope that helps,
-- 
The best we can hope for concerning the people at large is that they
be properly armed.  -- Alexander Hamilton
-- Eric Hopper (hopper@omnifarious.org 
http://www.omnifarious.org/~hopper) --