A Case for a Working-set-based Memory Hierarchy

Modern microprocessor designs continue to obtain impressive per-formance gains through increasing clock rates and advances in the parallelism obtained via micro-architecture design. Unfortunately, corresponding improvements in memory design technology have not been realized, resulting in latencies of over 100 cycles between processors and main memory. This ever-increasing gap in speed has pushed the current memory-hierarchy approach to its limit. Traditional approaches to memory-hierarchy management have not yielded satisfactory results. Hardware solutions require more power and energy than desired and do not scale well. Compiler so-lutions tend to miss too many optimization opportunities because of limited compile-time knowledge of run-time behavior. This pa-per explores a different approach that combines both approaches by making use of the static knowledge obtained by the compiler in the dynamic decision making of the micro-architecture. We propose a memory-hierarchy design based on working sets that uses compile-time annotations regarding the working set of memory operations to guide cache placement decisions. Our experiments show that a working-set-based memory hier-archy can significantly reduce the miss rate for memory-intensive tiled kernels by limiting cross interference. The working-set-based memory hierarchy allows the compiler to tile many loops without concern for cross interference in the cache, making tile size choice easier. In addition, the compiler can more easily tailor tile choices to the separate needs of different working sets