On Thu, Jan 9, 2020 at 5:03 AM 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. > > > - ==p1== is scheduled and its execution starts to wait on the semaphore, > so it is removed from the run queue of the scheduler and added to the > waiting list of the semaphore. > - ==p2== is scheduled and it signals the semaphore. The semaphore takes > the first waiting process (==p1==) and reschedule it by adding it to the > end of the suspended lists. > Since Smalltalk does not have a preemptive scheduler, neither p1 nor p2 will start to run until something else happens after the execution of p1 := [...] fork. p2 := [...] fork. So for example, if there is Processor yield then p1 can start to run. So you need to add code to your example to be able to determine what will happen. The easiest thing would be to delay long enough that both can run. 1 millisecond is more than enough. > Now this sentence "The semaphore takes the first waiting process (==p1==) > and reschedule it by adding it to the end of the suspended lists.” is super > naive. Is the semaphore signalling scheduled? or not? > I would say these three things, something like this: "A semaphore is a queue (implemented as a linked list) and an excess signals count, which is a non-negative integer. On instance creation a new semaphore is empty and has a zero excess signals count. A semaphore created for mutual exclusion is empty and has an excess signals count of one." "When a process waits on a semaphore, if the semaphore's excess signals count is non-zero, then the excess signal count is decremented, and the process proceeds. But if the semaphore has a zero excess signals count then the process is unscheduled and added to the end of the semaphore, after any other processes that are queued on the semaphore." "When a semaphore is signaled, if it is not empty, the first process is removed from it and added to the runnable processes in the scheduler. If the semaphore is empty its excess signals count is incremented. Given these three statements it is easy to see how they work, how to use them for mutual exclusion, etc. > > signal > "Primitive. Send a signal through the receiver. If one or more > processes > have been suspended trying to receive a signal, allow the first > one to > proceed. If no process is waiting, remember the excess signal. > Essential. > See Object documentation whatIsAPrimitive." > > <primitive: 85> > self primitiveFailed > > "self isEmpty > ifTrue: [excessSignals := excessSignals+1] > ifFalse: [Processor resume: self removeFirstLink]" > > > I wanted to know what is really happening when a semaphore is signalled. > Now resume: does not exist on Processor. > > I will look in the VM code. > > > S > > > > > > S. > > -- _,,,^..^,,,_ best, Eliot
