Split hardware transactions: true nesting of transactions using best-effort hardware transactional memory

Transactional Memory (TM) is on its way to becoming the programming API of choice for writing correct, concurrent, and scalable programs. Hardware TM (HTM) implementations are expected to be significantly faster than pure software TM (STM); however, full hardware support for true closed and open nested transactions is unlikely to be practical. This paper presents a novel mechanism, the split hardware transaction (SpHT), that uses minimal software support to combine multiple segments of an atomic block, each executed using a separate hardware transaction, into one atomic operation. The idea of segmenting transactions can be used for many purposes, including nesting, local retry, orElse, and user-level thread scheduling; in this paper we focus on how it allows linear closed and open nesting of transactions. SpHT overcomes the limited expressive power of best-effort HTM while imposing overheads dramatically lower than STM and preserving useful guarantees such as strong atomicity provided by the underlying HTM.