itsy bitsy Unix

[Huw Davies]

On 18/03/2005, at 12:36 PM, Paul Koning wrote:

>>>>>> "Huw" == Huw Davies <huw.davies at kerberos.davies.net.au> writes:
>
>  Huw> Anyone who's used a Macintosh before OS-X might want to disagree
>  Huw> here.
>
> Depends on what you're doing.
>
> If you want to run large workstation applications, then you want a
> real Unix-style OS.
>
> The original question was for a very small multitasking system, which
> to me sounded more like an embedded/RTOS type of application.  That's
> where the cooperative style often works better.

I suspect a lot depends on how much control over the total environment 
you have.

For a small embedded/RTOS you probably have (or at least should have) 
total
control. If you don't you're depending on the writer of one (or more) 
of the
processes "doing the right thing". Now probably they will try to do 
this, but
any subtle bug may lead to one of the processes failing to relinquish 
the CPU
and then "bad things happen (TM)" - this was the typical Mac experience.

Of course, having said that, Macs worked remarkably well for many years.

> One benefit of cooperative, rather than preemptive, scheduling is that
> the scheduling properties and liveness properties of the various tasks
> are very obvious.  If reliability demands that everything gets a slice
> of time, then cooperative multitasking, without priority, is a great
> way to get the job done.

Are the risks in preemptive scheduling just ones of complexity? I guess
that it is much harder to "prove" that this type of scheduler is going
to to the right thing at the right time. I know that,
for example, the scheduler for VMS has gone from a couple of pages of 
assembler
to something like 75 pages of BLISS!
>
Huw Davies           | e-mail: Huw.Davies at kerberos.davies.net.au
Melbourne            | "If soccer was meant to be played in the
Australia            | air, the sky would be painted green"