Efficient Synchronization: Let Them Eat QOLB /sup1/

Abstract

Efficient synchronization primitives are essential for achieving high performance in fine-grain, shared-memory parallel programs. One function of synchronization primitives is to enable exclusive access to shared data and critical sections of code. This paper makes three contributions. (1) We enumerate the five sources of overhead that locking synchronization primitives can incur. (2) We describe four mechanisms (local spinning, queue-based locking, collocation, and synchronized prefetch) that reduce these synchronization overheads. (3) With detailed simulations, we show the extent to which these four mechanisms can improve the performance of shared-memory programs. We evaluate the space of these mechanisms using seventeen synchronization constructs, which are formed from six base typed of locks (TEST&SET, TEST&TEST&SET, MCS, LH, M, and QOLB). We show that large performance gains (speedups of more than 1.5 for three of five benchmarks) can be achieved if at least three optimizing mechanisms are used simultaneously. We find that QOLB, which incorporates all four mechanisms, outperforms all other primitives (including reactive synchronization) in all cases. Finally, we demonstrate the superior performance of a low-cost implementation of QOLB, which runs on an unmodified cluster of commodity workstations.