How to block enqueue?

[poor1017]

Hi @cameron314

Thank you for your greate library!

I have a concurrent queue, the producer will generate UUID then enqueue, several consumers will take UUID from the queue. My requirement is: if the queue has enough UUIDs (we can call this as high water level) in it, it will block enqueue, until its UUIDs is lower than a number (we can call this low water level).

If high/low water level is too complex, what about a threshold?

Looking forward to your reply.

[cameron314]

Sounds like you want to use a semaphore in front of the queue. A LightweightSemaphore is included in this library.

[poor1017]

@cameron314

Thank you for your suggestion. I think I can implement like this:

1. Consumers call size_approx() every time they get a UUID;
2. If size_approx is lower than the low water mark, then sema->signal() is called in consumer thread to notify the producer to continue producing UUIDs;
3. What I don't know is how to make the producer stop producing UUIDs and block it if size_approx is greater than the high water mark?
4. In addition, consumers have to call size_approx every time they get a UUID. Will the performance be greatly affected?

Looking forward to your reply.

[cameron314]

Don't use size_approx, you'll have races.

Set the semaphore to the maximum number of items. Before the producer enqueues, wait on the semaphore. After a consumer dequeues, signal the semaphore.

There will be some overhead, but not much. LightweigthSemaphore is designed to be very efficient in the hot path when no blocking is actually necessary.

[poor1017]

Hi @cameron314

1. Before the producer enqueues, wait on the semaphore -> the queue is always empty;
2. After a consumer dequeues, signal the semaphore -> the consumer is always blocking;

Is this a dead-lock?

[cameron314]

I don't understand. Why would the queue stay empty? The first N waits on the semaphore would return immediately.

Also, signaling a semaphore is not a blocking operation.

[poor1017]

@cameron314

Before the producer enqueues, wait on the semaphore.

The queue is empty at beginning, and the semaphore will never get signal.

After a consumer dequeues, signal the semaphore.

Because the queue is always empty, so dequeuing is blocking, so signaling operation can not be excuted.

[cameron314]

The semaphore would be initialized to some value larger than 0.

[poor1017]

@cameron314

Your solution works very well, thank you!