Compiler Directed Speculative Intermittent Computation

TL;DR

CoSpec introduces a compiler-architecture co-design for energy-harvesting in-order processors that uses speculation and region partitioning to ensure crash consistency and improve performance during power outages.

Contribution

It proposes a novel speculative approach with region-based partitioning to achieve crash consistency without unconventional hardware support in energy-harvesting systems.

Findings

Achieves over 1.2X speedup in no-outage scenarios.

Outperforms state-of-the-art by 1.8~3X on energy harvesting traces.

Effectively manages power failures with region-based speculation.

Abstract

This paper presents CoSpec, a new architecture/compiler co-design scheme that works for commodity in-order processors used in energy-harvesting systems. To achieve crash consistency without requiring unconventional architectural support, CoSpec leverages speculation assuming that power failure is not going to occur and thus holds all committed stores in a store buffer (SB), as if they were speculative, in case of mispeculation. CoSpec compiler first partitions a given program into a series of recoverable code regions with the SB size in mind, so that no region overflows the SB. When the program control reaches the end of each region, the speculation turns out to be successful, thus releasing all the buffered stores of the region to NVM. If power failure occurs during the execution of a region, all its speculative stores disappear in the volatile SB, i.e., they never affect program states in NVM. Consequently, the interrupted region can be restarted with consistent program states in the wake of power failure. To hide the latency of the SB release, i.e., essentially NVM writes, at each region boundary, CoSpec overlaps the NVM writes of the current region with the speculative execution of the next region. Such instruction level parallelism gives an illusion of out-of-order execution on top of the in-order processor, achieving a speedup of more than 1.2X when there is no power outage. Our experiments on a set of real energy harvesting traces with frequent outages demonstrate that CoSpec outperforms the state-of-the-art scheme by 1.8~3X on average.