A User-Guided Locking API for the OpenMP* Application Program Interface

Although the OpenMP API specification defines a set of runtime routines for simple and nested locks, there is no standardized way to select different lock implementations. Programmers have to use vendor extensions to globally alter the lock implementation for the application; fine-grained control is not possible. Proper use of hardware-based speculative locks can achieve significant runtime improvements but, if used inappropriately, they can lead to severe performance penalties. Thus programmers need to be able to explicitly choose the right lock implementation on a per-lock basis. In this paper, we extend the OpenMP API for locks with functions to provide such hints to the implementation. We also extend the syntax and semantics of the critical construct with clauses to contain hints. Our performance results for micro-benchmarks show that the runtime selection of lock implementations does not add any noticeable overhead. We also show that using an appropriate runtime hint can improve application performance.