Done.
https://github.com/pharo-project/pharo/issues/5466
It was so simple that I preferred to do it myself so like that I will look like
a great Pharo contributor.
> On 10 Jan 2020, at 06:30, Danil Osipchuk <[email protected]> wrote:
>
> Maybe to add this into the class comment, this is the most concise and clear
> description of how it works i've ever seen
>
> пт, 10 янв. 2020 г., 8:13 Eliot Miranda <[email protected]
> <mailto:[email protected]>>:
>
>
> On Thu, Jan 9, 2020 at 5:03 AM ducasse <[email protected]
> <mailto:[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.
>
>
> - ==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
