Semaphore Queues FIFO?

[Michail Turovskiy]

For the semaphore queues, is there any restriction on them being FIFO?

When processes get added to a particular semaphore's wait queue, should
we preserve the order they came in so that the process that called P()
next will be the one that gets the semaphore? Or can we just pick the
process with the highest priority to get the semaphore, regardless of
the actual ordering?


Thanks,
m.t.

[Iulian Neamtiu]

Post by Michail Turovskiy
For the semaphore queues, is there any restriction on them being FIFO?

Well, FIFO is actually a requirement :) You don't have to
do anything, just using Wait and Wake_Up[_One] in the implementation
will take care of it, since Wait/Wake_up ensure a FIFO policy.
Priority does not matter at all, only ordering matters.


Iulian

Post by Michail Turovskiy
When processes get added to a particular semaphore's wait queue, should
we preserve the order they came in so that the process that called P()
next will be the one that gets the semaphore? Or can we just pick the
process with the highest priority to get the semaphore, regardless of
the actual ordering?
Thanks,
m.t.

[Michael Hicks]

Post by Iulian Neamtiu

Post by Michail Turovskiy
For the semaphore queues, is there any restriction on them being FIFO?

Well, FIFO is actually a requirement :) You don't have to
do anything, just using Wait and Wake_Up[_One] in the implementation
will take care of it, since Wait/Wake_up ensure a FIFO policy.
Priority does not matter at all, only ordering matters.

As a quick clarification: Wake_Up_One actually uses Find_Best, which
takes the first thread of those with the highest priority. However,
since only user processes will be on these queues, they all have the
same priority, so it boils down to FIFO.

-Dr. Hicks