Scaling concurrent queues by using HTM to profit from failed atomic operations

Abstract

Queues are fundamental concurrent data structures, but despite years of research, even the state-of-the-art queues scale poorly. This poor scalability occurs because of contended atomic read-modify-write (RMW) operations. This paper makes a first step towards designing a scalable linearizable queue. We leverage hardware transactional memory (HTM) to design TxCAS, a scalable compare-and-set (CAS) primitive—despite HTM being targeted mainly at uncontended scenarios. Leveraging TxCAS’s scalability requires a queue design that does not blindly retry failed CASs. We thus apply TxCAS to the baskets queue, which steers enqueuers whose CAS fails into dedicated basket data structures. Coupled with a new, scalable basket algorithm, we obtain SBQ, the scalable baskets queue. At high concurrency levels, SBQ outperforms the fastest queue today by 1.6× on a producer-only workload.