Design Issues and Tradeoffs for Write Buffers

Processors with write-through caches typically require a write buffer to hide the write latency to the next level of memory hierarchy and to reduce write traffic. A write buffer can cause processor stalls when it is full, when it contends with a cache miss for access to the next level of the hierarchy, and when it contains the freshest copy of data needed by a load. This paper uses instruction-level simulation of SPEC92 benchmarks to investigate how different write buffer depths, retirement policies, and load-hazard policies affect these three types of write-buffer stalls. Deeper buffers with adequate headroom, lazier retirement policies, and the ability to read data directly from the write buffer combine to substantially reduce write-buffer-induced stalls.