Two-Phase Barrier: A Synchronization Primitive for Improving the Processor Utilization

Barrier is widely used for synchronization in parallel programs. Since the process arrived earlier than others should wait at the barrier, the total processor utilization decreases. In this paper, to find the sources of the barrier waiting time, parallel programs are executed on the various grain sizes through execution-driven simulations. In simulation studies, we found that even if approximately equal amounts of work are distributed to each processor, all processes may not arrive at a barrier at the same time. The reasons are that the different numbers of cache misses and instructions within in partitioned grains result in the difference in arrival time of processors at the barrier. In this paper, the two-phased barrier is considered to reduce the blind waiting time in the traditional barrier scheme, which can be simply constructed by dividing one specific stage for the synchronization into two stages. On each stage, processes decide their stall or not, which is dependent on the current execution state of grains running on any given processors. Simulation results show that the reduced barrier waiting times attributed to the two-phased barrier contribute to the performance improvement of parallel programs.