Reducing the Write Traffic for a Hybrid Cache Protocol

Coherence misses limit the performance of write-invalidate cache protocols in large-scale shared-memory multi-processors. By contrast, hybrid protocols mix updates with invalidations and can reduce the coherence miss rate. The gains of the fewer coherence misses, however, can sometimes be outweighed by contention due to the extra traffic making techniques to cut the write traffic important. We study in this paper how write traffic for hybrid protocols can be reduced by incorporating a write cache in each node. Detailed architectural simulations reveal that write caches are effective in exploiting locality in write accesses under relaxed memory consistency models. Hybrid protocols augmented with write caches with only a few entries are shown to outperform a write-invalidate protocol for all five benchmark applications under study.