Multithreading and up-to-date check

Asked Sep 28 '21 at 18:21

Active Sep 28 '21 at 18:21

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Does the following require some kind of synchronization?

Control	Worker 1	Worker 2	...	Worker n
Test if all a_n is true. If so, dispatch a new job. Otherwise, try again.	when done: set a_1 to true	when done: set a_2 to true	...	when done: set a_n to true

The test performed by the control thread is not atomic. However, a_n can never be set to false. Thus, it is guaranteed that all a_n will be true for some iteration in the control thread. Is synchronization still necessary?

This control flow is used to process jobs in a graph. All tasks is stored in a linked list. When all $a_{kn}$ are true, job $k$ is dispatched and removed from the list. If they are not, try $a_{k+1}$. When the end of the list is reached, start over from the beginning of the list. Continue until the list is empty.

Note: I assume that the graph is a DAG.

asked Sep 28 '21 at 18:21

user877329

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If control thread reads variable `a_n` and there is no synchronization requested, then it could cache variable value for as long as it wants. When worker thread will change variable to true then control thread may never see this change. – Alexander Pavlov Sep 28 '21 at 19:48
In Java it would be enough to mark `a_n` variable as `volatile` (or use `AtomicBoolean`) – Alexander Pavlov Sep 28 '21 at 19:52
@AlexanderPavlov But atomic should be sufficient – user877329 Sep 29 '21 at 15:21
for your task even volatile is sufficient. `AtomicBoolean` just provides additional handy methods – Alexander Pavlov Sep 29 '21 at 17:58

Multithreading and up-to-date check

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