This isn't really a great solution though, since it pushes the problem of ensuring FIFO ordering outside of the queue and onto the user; but being FIFO is entirely the point of the queue data structure. If your queue doesn't provide ordering across multiple threads, it can't meaningfully be called a multi-threaded queue, so it's not very fair to compare it to genuine multi-threaded queues. Unless I've misread the documentation, this is just a collection of SPMC queues, together with a read operation that checks all the queues for an item to dequeue, right? But there is no synchronization that occurs to tie any of these SPMC queues together. Even if you wrapped every single access to this data structure in a lock, for instance, you would not be able to ensure FIFO ordering of elements inserted into it.

I think the estimation that having inter-thread ordering requirements is uncommon is incorrect. Probably the most common use of MPMC queues is as a concurrency primitive in systems using a CSP-like abstraction, where such requirements are extremely common. But you can't implement, say, Go's channels in terms of this data structure.