On the effectiveness of speculative and selective memory fences

Memory fences inhibit the reordering of memory accesses in modern microprocessors; fences are useful to implement synchronization and strong shared memory semantics in multi-threaded programs. A naive implementation of memory fences can result in a significant performance penalty for processors with deep pipelines supporting multiple concurrent memory accesses. The paper compares three techniques to reduce the impact of memory fences: (1) Read-speculation allows reads that follow a fence to be issued while the fence is being processed; (2) Write-ahead additionally allows writes following a fence to proceed early; (3) Selective fences distinguish between memory accesses to thread-local and shared memory and enforce ordering only among accesses to shared memory. We evaluate and compare the effectiveness of these techniques with a simulator derived from the Pentium 4 architecture. We report data for a storage model that uses memory fences to enforce the memory semantics at monitor boundaries.