Data Forwarding in Scalable Shared-Memory Multiprocessors1

Scalable shared-memory multiprocessors are often slowed down by long-latency memory accesses. One way to cope with this problem is to use data forwarding to overlap memory accesses with computation. With data forwarding, when a processor produces a datum, in addition to updating its cache, it sends a copy of the datum to the caches of the processors that the compiler identified as consumers of it. As a result, when the consumer processors access the datum, they find it in their caches. This paper addresses two main issues. First, it presents a framework for a compiler algorithm for forwarding. Second, using address traces, it evaluates the performance impact of different levels of support for forwarding. Our simulations of a 32-processor machine show that, on average, a slightly-optimistic support for forwarding speeds up five applications by 50% for large caches and 30% for small caches. For large caches, most read sharing misses can be eliminated, while for small caches, forwarding rarely increases the number of conflict misses. Overall, support for forwarding in shared-memory multiprocessors promises to deliver good application speedups.