Limit max number of elements in a queue

[iefserge]

Thank you for this great library!

I was wondering if it's possible to limit a maximum number of elements in a queue (possibly per producer, at compile time)? In some cases it's necessary to prevent unlimited queue growth. I know I can override memory allocation callbacks but maybe there is a simpler way.

[cameron314]

Hmm, interesting. There's no nice way of doing that right now (sorry!). I'll have to think about it.

In the meantime, here's a (hackish) workaround:
First, use only explicit producers (because they never relinquish their blocks).
Then, before unleashing your threads, enqueue and dequeue exactly the per-producer maximum number of items (use a multiple of the block size) from each producer. This will pre-fill the producers' block lists internally. Then, if you use only try_enqueue with those pre-allocated producer tokens, the operation will fail if there's not enough room in the producer, achieving the per-producer maximum size guarantee, albeit with significant effort and limitations.

(You can set the initial size of the queue (constructor argument) to the sum total of the producer sizes to reduce dynamic allocation in this workaround.)

[iefserge]

@cameron314 Thanks, I'll try that.

[cameron314]

I've added a MAX_SUBQUEUE_SIZE trait. It rounds up to the next block size boundary, and is checked at the block level (so as not to introduce any extra overhead in the fast path); if an enqueue operation would cause a block to be allocated that could cause the size of that producer's sub-queue to exceed the rounded maximum, the enqueue operation will gracefully fail (this includes bulk enqueue operations).

Cheers!

[iefserge]

Thank you, very cool!