Nonblocking transactions without indirection using alert-on-update

Nonblocking implementations of software transactional memory (STM) typically impose an extra level of indirection when accessing an object; some researchers have claimed that the cost of this indirection outweighs the semantic advantages of nonblocking progress guarantees. We consider this claim in the context of a simple hardware assist, alert-on-update (AOU), which allows a thread to request immediate notification if specified line(s) are replaced or invalidated in its cache. We show that even a single AOU line allows us to construct a simple, nonblocking STM system without extra indirection. At the same time, we observe that per-load validation operations, required for intra-object consistency in both the new system and in lock-based (blocking) STM, at least partially negate the resulting performance gain. Moreover, inter-object consistency checks, also required in both kinds of systems, remain the dominant cost for transactions that access many objects. We therefore present a second nonblocking STM system that uses multiple AOU lines (one per accessed object) to eliminate validation overhead entirely, resulting in a nonblocking, zero-indirection STM system that outperforms competing systems by as much as a factor of 2.