Fast barriers for scalable ccNUMA systems

The contributions of this paper are threefold. First, we identify and quantify the performance deficiencies of conventional barrier implementations when they are executed on real (non-idealized) hardware. Second, we propose a queue-based barrier algorithm that has effectively O(1) time complexity as measured in round trip message latencies. Third, we demonstrate how matching the barrier implementation to the way that modern shared memory systems operate can improve performance dramatically by exploiting a hardware write-update (PUT) mechanism for signaling. The resulting barrier algorithm only costs one serialized round trip message latency to perform a barrier operation across N processors. Using a cycle-accurate execution-driven simulator of a future-generation SGI multiprocessor, we show that with no special hardware support our queue-based barrier outperforms OpenMP´s LL/SC-based barrier implementation by a factor of 7.9 on 256 processors. With hardware that supports a coherent PUT operation, our queue-based barrier outperforms OpenMP barriers by a factor of 94 and outperforms barriers based on SGI´s memory controller-based atomic operations by a factor of 6.5 on 256 processors.