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From: bill@mwca.UUCP (Bill Sheppard)
Newsgroups: comp.sys.atari.st
Subject: Re: Multitasking
Message-ID: <1809@mwca.UUCP>
Date: 18 Feb 91 18:05:14 GMT
References: <1991Feb11.151210.4010@informatik.uni-erlangen.de> <1976@lee.SEAS.UCLA.EDU> <1991Feb14.133758.3687@doe.utoronto.ca> <1986@lee.SEAS.UCLA.EDU>
Organization: Microware Systems Corp., Santa Clara, CA
Lines: 30

[discussion of various multitasking systems deleted]
>Other implementations out there for other machines don't shine so
>brightly.  (I won't name names in the interest of not throwing spoilt 
>apples at other companies, but...) Have you ever used a presumably 
>fast machine that used cooperative multitasking? The 40% overhead figure 
>that I mentioned above would not seem unrealistic to you after using one.

A consultant doing a study for a customer of ours did some research into
the amount of overhead our operating system (OS-9) required; his findings
were that on a 20 MHz 68020 the the OS required less than 0.5% of the CPU's
processing bandwidth, using a 10 ms clock tick, 2 ticks/slice (which equals
50 time slices/second).  This overhead consisted primarily of the scheduler
aging processes and saving context/switching process where appropriate.
This was not on an ST, so OS tasks such as mouse handling weren't needed.
A 68030 should require substantially less overhead, since as the processor
power goes up the relative amount of time needed to do housekeeping goes
down.  Also, the on-board cache of a 68030 should make a substantial
difference.  Of course, the same holds true in the other direction - a 68000
would require substantially more overhead as a percentage of total processing
bandwidth.  Also, OS-9 is a real-time operating system, and so is tuned to
require as little overhead as possible.  More general-purpose OS' such as
TOS (were it multi-tasking), Amiga OS, Mac System 7.0, and Unix generally
would require significantly more overhead (a factor of 10 wouldn't be an
unreasonable assumption, at least for Unix).

-- 
 ##############################################################################
 # Bill Sheppard  --  bills@microware.com  --  {uunet,sun}!mcrware!mwca!bill  #
 # Microware Systems Corporation  ---  OS-9: Seven generations beyond OS/2!!  #
 ######Opinions expressed are my own, though you'd be wise to adopt them!######