Monday, November 10, 2014

Detailed Design of a Lock-Free Queue

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This post outlines, in quite some detail, my design for an efficient lock-free queue that supports multiple concurrent producers and consumers (an MPMC queue). My C++11 implementation of this design can be found on GitHub. A more high-level overview of the queue, including benchmark results, can be found in my initial blog post introducing my implementation.

One key insight is that the total order that elements come out of a queue is irrelevant, as long as the order that they went in on a given thread matches the order they come out on another thread. This means that the queue can be safely implemented as a set of independent queues, one for each producer; writing a single-producer, multi-consumer lock free queue is much easier than writing a multi-producer, multi-consumer one, and can be implemented much more efficiently too. The SPMC queue can be generalized to an MPMC one by having the consumers pull from different SPMC queues as needed (and this can be done efficiently as well with some cleverness). A heuristic is used to speed up dequeueing in the typical case, pairing consumers with producers as much as possible (which drastically reduces the overall contention in the system).

Apart from the high-level set-of-SPMC-queues design, the other key part of the queue is the core queueing algorithm itself, which is brand new (I devised it myself) and unlike any other that I've heard of. It uses an atomic counter to track how many elements are available, and once one or more have been claimed (by incrementing a corresponding elements-consumed counter and checking whether that increment turned out to be valid), an atomic index can be safely incremented to get the actual IDs of the elements to reference. The problem is then reduced to mapping integer IDs to individual elements, without having to worry about other threads referencing the same objects (each ID is given to only one thread). Details are below!

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