The arrival of best-effort hardware transactional memory (TM) creates a challenge for designers of transactional memory runtime libraries. On the one hand, using hardware TM can dramatically reduce the latency of transactions. On the other, it is critical to create a fall-back path to handle the cases where hardware TM cannot complete a transaction, and this path ought to be scalable and reasonably fair to all transactions. Additionally, while the hardwareaccelerated system is likely to have

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... ker safety guarantees than a pure hardware TM, it ought not to be weaker than what software TM guarantees. We propose a new hybrid TM algorithm based on the "Cohorts" software TM algorithm. Our algorithm guarantees opacity by preventing any transaction from observing the un-committed state of any other transaction. It does so via a novel state machine that maximizes the use of hardware TM, while affording opportunity to enforce fairness policies. We present an implementation of our Hybrid Cohorts that prioritizes transactions that fall back to software mode. In this manner, we ensure that long-running transactions do not starve, while still allowing concurrency among hardware and software transactions.