itsy bitsy Unix

[jimmydevice]

Huw Davies wrote:

>
> 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"
>
>
The advantage of a preemptive scheduler is that a dead process will not 
block a task swich. This problem, as  observed by other  list members, 
should not occure in a properly designed system. But what isystem is 
properly designed after you have more than one developer working on it?
Jim Davis,