I/O Speculation for the Microsecond Era

Microsecond latencies and access times will soon dominate most datacenter I/O workloads, thanks to improvements in both storage and networking technologies. Current techniques for dealing with I/O latency are targeted for either very fast (nanosecond) or slow (millisecond) devices. These techniques are suboptimal for microsecond devices - they either block the processor for tens of microseconds or yield the processor only to be ready again microseconds later. Speculation is an alternative technique that resolves the issues of yielding and blocking by enabling an application to continue running until the application produces an externally visible side effect. State-of-the-art techniques for speculating on I/O requests involve checkpointing, which can take up to a millisecond, squandering any of the performance benefits microsecond scale devices have to offer. In this paper, we survey how speculation can address the challenges that microsecond scale devices will bring. We measure applications for the potential benefit to be gained from speculation and examine several classes of speculation techniques. In addition, we propose two new techniques, hardware checkpoint and checkpoint-free speculation. Our exploration suggests that speculation will enable systems to extract the maximum performance of I/O devices in the microsecond era.