Non-vital loads

Abstract

As the frequency gap between main memory and modern microprocessor grows, the implementation and efficiency of on-chip caches become more important. The growing latency to memory is motivating new research into load instruction behavior and selective data caching. This work investigates the classification of load instruction behavior. A new load classification method is proposed that classifies loads into those vital to performance and those not vital to performance. A limit study is presented to characterize different types of non-vital loads and to quantify the percentage of loads that are non-vital. Finally, a realistic implementation of the non-vital load classification method is presented and a new cache structure called the Vital Cache is proposed to take advantage of non-vital loads. The Vital Cache caches data for vital loads only, deferring non-vital loads to slower caches. Results: The limit study shows 75% of all loads are non-vital with only 35% of the accessed data space being vital for caching. The Vital Cache improves the efficiency of the cache hierarchy and the hit rate for vital loads. The Vital Cache increases performance by 17%.