Operating System I/O Speculation: How Two Invocations Are Faster Than One

We present an in-kernel disk prefetcher which uses speculative execution to determine what data an application is likely to require in the near future. By placing our design within the operating system, we provide several benefits compared to the previous application-level design. Not only is our system easier to implement and deploy, but by handling page faults as well as traditional file-access methods we are able to apply speculative execution to swapping applications, which often spend the majority of their execution time fetching non-resident pages. We also present two new OS features that further improve the performance of speculative execution for applications that have large page tables and working sets. These are a fast method for synchronizing an errant speculative process with normal execution, and a modified form of copy-on-write which preserves application semantics without delaying normal execution. Finally, by leveraging OS knowledge about memory usage and contention, we design a mechanism for estimating and limiting the memory overhead of speculative executions.

Our implementation for Linux 2.4.8 provides benefits of up to 60% for a wide range of explicit-I/O and swapping applications. Our results show that our support mechanisms for swapping applications provide significant performance benefits, and in some cases prevent speculative execution from hurting performance. We further demonstrate that our memory control mechanism effectively limits speculative overheads while allowing beneficial speculative executions to proceed unhindered.