[Koromix]

The reader scans all queues, without locking. If one is non-empty, then you lock it and pop an item. Only if are all queues are empty then you hit the slow path and wait on a synchronization primitive (condition variable).

Of course if you can assume that the queue will always be very busy, you can ditch that too and poll instead. Polling is often bad, but not always. When things go very fast, it can be more efficient.

A good example of that occurs with the new storage NVM Express storage technologies and Linux, where the traditional I/O completion interrupt used until now is starting to bottleneck: https://lwn.net/Articles/663879/