Techniques for Efficient Processing in Runahead Execution Engines

Runahead execution is a technique that improves proces-sor performance by pre-executing the running application instead of stalling the processor when a long-latency cache miss occurs. Previous research has shown that this tech-nique significantly improves processor performance. How-ever, the efficiency of runahead execution, which directly affects the dynamic energy consumed by a runahead pro-cessor, has not been explored. A runahead processor exe-cutes significantly more instructions than a traditional out-of-order processor, sometimes without providing any per-formance benefit, which makes it inefficient. In this paper, we describe the causes of inefficiency in runahead execu-tion and propose techniques to make a runahead processor more efficient, thereby reducing its energy consumption and possibly increasing its performance. Our analyses and results provide two major insights: (1) the efficiency of runahead execution can be greatly im-proved with simple techniques that reduce the number of short, overlapping, and useless runahead periods, which we identify as the three major causes of inefficiency, (2) sim-ple optimizations targeting the increase of useful prefetches generated in runahead mode can increase both the perfor-mance and efficiency of a runahead processor. The tech-niques we propose reduce the increase in the number of in-structions executed due to runahead execution from 26.5% to 6.2%, on average, without significantly affecting the per-formance improvement provided by runahead execution. 1