Transactional Read-Modify-Write Without Aborts

Abstract

Language-level transactions are said to provide “atomicity,” implying that the order of operations within a transaction should be invisible to concurrent transactions and thus that independent operations within a transaction should be safe to execute in any order. In this article, we present a mechanism for dynamically reordering memory operations within a transaction so that read-modify-write operations on highly contended locations can be delayed until the very end of the transaction. When integrated with traditional transactional conflict detection mechanisms, our approach reduces aborts on hot memory locations, such as statistics counters, thereby improving throughput and reducing wasted work. We present three algorithms for delaying highly contended read-modify-write operations within transactions, and we evaluate their impact on throughput for eager and lazy transactional systems across multiple workloads. We also discuss complications that arise from the interaction between our mechanism and the need for strong language-level semantics, and we propose algorithmic extensions that prevent errors from occurring when accesses are aggressively reordered in a transactional memory implementation with weak semantics.