Conflict Reduction in Hardware Transactions Using Advisory Locks

Preliminary experience with hardware transactional memory suggests that aborts due to data conflicts are one of the principal obstacles to scale-up. To reduce the incidence of conflict, we propose an automatic, high-level mechanism that uses advisory locks to serialize (just) the portions of the transactions in which conflicting accesses occur. We demonstrate the feasibility of this mechanism, which we refer to as staggered transactions, with fully developed compiler and runtime support,running on simulated hardware. Our compiler identifies and instruments a small subset of the accesses in each transaction, which it determines, statically, are likely to constitute initial accesses to shared locations. At run time, the instrumentation acquires an advisory lock on the accessed datum, if (and only if) prior execution history suggests that the datum---or locations``downstream'' of it---are indeed a likely source of conflict. Policy to drive the decision requires one hardware feature not generally found in current commercial offerings: nontransactional loads and stores within transactions. It can also benefit from a mechanism to record the program counter at which a cache line was first accessed in a transaction. Simulation results show that staggered transactions can significantly reduce the frequency of conflict aborts and increase program performance.