On 11 April 2006 22:24, John Meacham wrote:
I'd like to be a bit more formal when it comes to the distinction between cooperative and preemptive implementations of concurrency, here is a first attempt.
1. termination,
In a concurrent implementation, a thread performing an infinite loop with no IO or interaction with the outside world can potentially stall switching to another thread forever, in FP, we usually denote an infinite loop by _|_. so I think the first difference would be:
[Rule 1] * in a cooperative implementation of threading, any thread with value _|_ may cause the whole program to have value _|_. In a preemtive one, this is not true.
I don't know what it means for a thread to "have value _|_". A thread is defined by its observable effects, threads don't have values.
I am using _|_ in the stronger sense of non-termination, not including things like exceptions which should have a well defined behavior.
2. fairness
Assuming no thread has value _|_ now, what can both models guarentee when it comes to fairness?
both can guarentee every runnable thread will eventually run with a round-robin type scheduler.
What if one of the threads never yields in a cooperative system? Even if it isn't calculating _|_, if it's just endlessly doing some pointless IO? Cheers, Simon