Hi Alistair, > On 12 Jan 2020, at 09:33, Alistair Grant <[email protected]> wrote: > > On Thu, 9 Jan 2020 at 13:01, ducasse <[email protected]> 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.
I don't understand, in your second paragraph you say 'Pharo has #processPreemptionYields set to true' and now you say it should become the default. Is that already the case or not then ? > Running the following variant, and then typing in to another window, > demonstrates the behaviour: I am not sure what you want to demonstrate: that it is totally random depending on external factors ;-) ? Which is pretty bad: how should semaphores be used (safely) ? What are good examples of real world correct semaphore usage ? Right now, all the explanations around scheduling of processes and their priorities make it seem as if the answer is 'it all depends' and 'there is no way to be 100% sure what will happen'. Sven > | 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
