Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux

Fast userlevel locking is an alternative locking mechanism to the typically heavy weight kernel approaches such as fcntl locking and System V semaphores. Here, multiple processes communicate locking state through shared memory regions and atomic operations. Kernel involvement is only necessary when there is contention on a lock, in order to perform queueing and scheduling functions. In this paper we discuss the issues related to user level locking by following the history of ideas and the code to the current day. We present the efficacy of "futexes" through benchmarks, both synthetic and through adaptations to existing databases. We conclude by presenting the potential future directions of the "futex" interface.