Write Prediction for Persistent Memory Systems

Abstract

Persistent memories (PMs) provide byte-addressable persistent storage with an access latency close to DRAM. Programs can achieve better performance by directly managing persistent data in PM. Unfortunately, these programs need to explicitly order in which data becomes persistent to PM for recoverability. Such enforced orderings place the write-back latency on the critical path of the execution. On the other hand, PM systems integrate different operations to support security, bandwidth reduction, endurance, etc. These PM-support operations further increase the write latency as they first transform data (e.g., encrypt and compute integrity tree) before persisting it to PM. Prior work proposed to use software hints to pre-execute these operations but can increase programmers' burden. The goal of this work is to mitigate the overhead due to PM-support operations in a software-transparent manner. If we can determine the address and value of a PM write-back ahead of time, it is possible to precompute its PM-support operations and move them off the critical path. We observe that both the address and data of PM write-backs often appear as values in prior store-instructions during the execution. For example, a PM allocator first uses a store-instruction to assign the allocated address to a pointer; later, the follow-up procedure updates that allocated address with new values and performs a write-back to PM. In this example, the value carried by the PM allocator's store-instruction contains the address of the later PM write-backs. Likewise, the data values also often appear in prior store-instructions, e.g., when passing values to a PM update function. Therefore, it is possible to predict the address and data, and precompute the PM-support operations for future write-backs by tracking store-instructions. We propose PMWeaver that learns the correlation between a PM write-back and the associated store-instructions and weaves together the address and data value of a PM write-back from the correlated store-instructions. Our evaluation demonstrates that PMWeaver correctly predicts 81.16% of addresses and 49.90% of data of PM write-backs in ten PM workloads, and yields 1.63× and 1.26× speedup over a no-prediction baseline system by precomputing two types of PM-support operations: a combination of encryption and integrity verification, and deduplication.