Transactional event profiling in a best-effort hardware transactional memory system

Introduction: Blue Gene/Q's (BG/Q) unique transactional memory system provides hardware isolation, atomicity and consistency for memory locations while leaving the details of the transactional programming system to software layers above the hardware [3]. This design allows for complex systems implemented as part of the software runtime. Here a profiling extension to the software runtime is presented. Motivation: In TM systems, conflict resolution refers to the choice of which transaction will abort when two transactions conflict. In BG/Q it is the younger transaction that always aborts, and is decided by the hardware. Once a rollback is initiated after abortion, the BG/Q runtime system becomes involved and makes a decision to either retry the aborted transaction speculatively, or to execute an operation called serialization. A serialized transaction is forced to completion on retry, possibly at the expense of otherwise non-conflicting transactions, which will abort themselves if they conflict or attempt to commit.