CRISP: critical slice prefetching

Abstract

The high access latency of DRAM continues to be a performance challenge for contemporary microprocessor systems. Prefetching is a well-established technique to address this problem, however, existing implemented designs fail to provide any performance benefits in the presence of irregular memory access patterns. The hardware complexity of prior techniques that can predict irregular memory accesses such as runahead execution has proven untenable for implementation in real hardware. We propose a lightweight mechanism to hide the high latency of irregular memory access patterns by leveraging criticality-based scheduling. In particular, our technique executes delinquent loads and their load slices as early as possible, hiding a significant fraction of their latency. Furthermore, we observe that the latency induced by branch mispredictions and other high latency instructions can be hidden with a similar approach. Our proposal only requires minimal hardware modifications by performing memory access classification, load and branch slice extraction, as well as priority analysis exclusively in software. As a result, our technique is feasible to implement, introducing only a simple new instruction prefix while requiring minimal modifications of the instruction scheduler. Our technique increases the IPC of memory-latency-bound applications by up to 38% and by 8.4% on average.