On Thu, 9 Jan 2020 at 13:01, ducasse <steph...@netcourrier.com> wrote: > > Hi > > I wanted to explain > > | semaphore p1 p2 | > semaphore := Semaphore new. > p1 := [ semaphore wait. > 'p1' crTrace ] fork. > > p2 := [semaphore signal. > 'p2' crTrace ] fork. > > displays p2 and p1. > but I would like explain clearly but it depends on the semantics of signal.
The way this is phrased seems to imply that 'p2' will always be displayed before 'p1', however in Pharo this is not guaranteed (when the processes are at the same priority, as they are this example). As Eliot implied in another reply, Pharo has #processPreemptionYields set to true, which means that any time a higher priority process preempts, the current process will be moved to the back of the queue. So in the case above, after p2 signals the semaphore, if a timer was delivered or keystroke pressed, p2 would be suspended and moved to the back of the queue. When the timer / keystroke / etc. had finished processing p1 would be at the front of the queue and would complete first. Since time and input events are (for practical purposes) unpredictable it means that the execution order of processes at a given priority is also unpredictable. While this isn't likely to happen in the example above, I have seen it regularly with TaskIt and multiple entries being run concurrently. I agree with Eliot that changing #processPreemptionYields to true by default would be an improvement in Pharo. It would make it easier to predict what is happening in a complex environment. Running the following variant, and then typing in to another window, demonstrates the behaviour: | semaphore p1 p2 | semaphore := Semaphore new. [ 100 timesRepeat: [ p1 := [ | z | semaphore wait. z := SmallInteger maxVal. 10000000 timesRepeat: [ z := z + 1 ]. 'p1' crTrace ] fork. p2 := [ | z | 1 second wait. semaphore signal. z := SmallInteger maxVal. 10000000 timesRepeat: [ z := z + 1 ]. 'p2' crTrace ] fork. 1 second wait. ] ] fork. The tail of transcript: 'p2' 'p1' 'p1' 'p1' 'p1' 'p2' 'p2' 'p2' 'p1' 'p1' 'p2' 'p1' 'p2' 'p2' 'p1' 'p1' 'p2' 'p1' Cheers, Alistair
