I-SPY: Context-Driven Conditional Instruction Prefetching with Coalescing

Modern data center applications have rapidly expanding instruction footprints that lead to frequent instruction cache misses, increasing cost and degrading data center performance and energy efficiency. Mitigating instruction cache misses is challenging since existing techniques (1) require significant hardware modifications, (2) expect impractical on-chip storage, or (3) prefetch instructions based on inaccurate understanding of program miss behavior. To overcome these limitations, we first investigate the challenges of effective instruction prefetching. We then use insights derived from our investigation to develop I-SPY, a novel profile-driven prefetching technique. I-SPY uses dynamic miss profiles to drive an offline analysis of I-cache miss behavior, which it uses to inform prefetching decisions. Two key techniques underlie I-SPY's design: (1) conditional prefetching, which only prefetches instructions if the program context is known to lead to misses, and (2) prefetch coalescing, which merges multiple prefetches of non-contiguous cache lines into a single prefetch instruction. I-SPY exposes these techniques via a family of light-weight hardware code prefetch instructions. We study I-SPY in the context of nine data center applications and show that it provides an average of 15.5% (up to 45.9%) speedup and 95.9% (and up to 98.4%) reduction in instruction cache misses, outperforming the state-of-the-art prefetching technique by 22.5%. We show that I-SPY achieves performance improvements that are on average 90.5% of the performance of an ideal cache with no misses.