Re: Complexity of protected objects

[Dale Stanbrough <dstanbro@bigpond.net.au>]

Robert Dewar wrote:

> Remember that the key point about protected objects is the
> ceiling priority protocol, which ensures that higher priority tasks
> can still freely interrupt the lower priority tasks inside a PO with a
> lower CP.

Just to clarify - the sentence above seems to imply that a low
priority task inside a PO can be forceably removed from the PO
by a higher priority task that want the -same- PO.

Of course this is impossible (if you want correct programs!). 
The reason you have POs is so that you can atomically modify
the state of an object. If you kick a task out 1/2 through, the
invariants for he PO (or the object it is protecting) could
easily be left in a corrupt state. Ceiling priority protocal
allows the high priority task to ensure it will be the -next-
task to get access to the PO, and to ensure that low priority 
tasks "hurry up and get out of the way" when working inside
a PO.

On the other hand if the higher priority task is resources
other than the PO used by the low priority tasks, then I agree.


> Thus it is
> quite reasonable to use complex PO's with a low CP for
> communication between low priority tasks without any
> danger of intefering with high priority tasks (even in
> an implementation that has no locks).

Another name for POs/semaphores are "bottlenecks". If you
want to increase the natural parallelism of a program, you
should make the code inside them as short as possible.


Dale