Location-based memory fences

Traditional memory fences are program-counter (PC) based. That is, a memory fence enforces a serialization point in the program instruction stream - - it ensures that all memory references before the fence in the program order have taken effect before the execution continues onto instructions after the fence. Such PC-based memory fences always cause the processor to stall, even when the synchronization is unnecessary during a particular execution. We propose the concept of location-based memory fences, which aim to reduce the cost of synchronization due to the latency of memory fence execution in parallel algorithms. Unlike a PC-based memory fence, a location-based memory fence serializes the instruction stream of the executing thread T₁ only when a different thread T₂ attempts to read the memory location which is guarded by the location-based memory fence. In this work, we describe a hardware mechanism for location-based memory fences, prove its correctness, and evaluate its potential performance benefit. Our experimental results are based on a software simulation of the proposed location-based memory fence, and thus expected to incur higher overhead than the proposed hardware mechanism would. Nevertheless, our software experiments show that applications can benefit from using location-based memory fences, but they do not scale as well in some cases, due to the software overhead. These results suggest that a hardware support for location-based memory fences is worth considering.