ABSTRACT Non-Blocking Timeout in Scalable Queue-Based Spin Locks

Queue-based spin locks allow programs with busy-wait synchronization to scale to very large multiprocessors, without fear of starvation or performance-destroying contention. Timeout-capable spin locks allow a thread to abandon its attempt to acquire a lock; they are used widely in real-time systems to avoid overshooting a deadline, and in database systems to recover from transaction deadlock and to tolerate preemption of the thread that holds a lock. In previous work we showed how to incorporate timeout in scalable queue-based locks. Technological trends suggest that this combination will be of increasing commercial importance. Our previous solutions, however, require a thread that is timing out to handshake with its neighbors in the queue, a requirement that may lead to indefinite delay in a preemptively multiprogrammed system. In the current paper we present new queue-based locks in which the timeout code is non-blocking. These locks sacrifice the constant worst-case space per thread of our previous algorithms, but allow us to bound the time that a thread may be delayed by preemption of its peers. We present empirical results indicating that space needs are modest in practice, and that performance scales well to large machines. We also conjecture that constant per-thread space cannot be guaranteed together with non-blocking timeout in a queuebased lock