Fast Checkpoint/Recovery to Support Kilo-Instruction Speculation and Hardware Fault Tolerance

The increased relative cost of accessing memory is encouraging processor designers to explore deeper uniprocessor speculation (e.g., with branch and value prediction) and consider multiprocessor speculation (e.g., on coherence message types and values). While some mechanisms have been proposed to support deep speculation using speculative multithreading, current mechanisms for conventional processors are not as good. To support kilo-instruction speculation with conventional processors, this paper proposes Multiversion Memory (MVM), a processor/memory interface that allows processors to create multiple versions of memory and recover to previous versions when necessary. In this paper, we develop an efficient implementation of MVM that uses a level one cache to keep recent speculative blocks (like a future file for memory), uses version buffers to keep old versions of blocks for which speculation is pending (like a memory history buffer), and leaves the level two cache (and beyond) unchanged (like a memory architectural file). Concurrently, requirements for highly-available computers and manufacturing trends to deepsub-micron design encourage techniques to mask transient faults (e.g., with error correcting codes and execution retry). Most current designs consider speculation and fault tolerance independently. Nevertheless, a second result of this paper is that MVM can provide support for both needs, perhaps making the use of hardware fault-tolerance more widespread. Simple cost models with parameters from commercial workloads show that our implementation of MVM allows kilo-instruction speculation and fault tolerance that can recover faster (e.g., less than 273 vs. 362 cycles), uses recovery storage that is smaller (e.g., 5,356 bytes vs. 10,000 bytes), and has lower common-case overhead than other recently proposed schemes.