Dynamic Identification of Shared Transactional Locations

Hardware TM systems execute user code within an atomic{} delimiter without any instrumentation. Software transactional memory systems require complex sequences of operations to be executed on the memory locations shared by transactions, but typically not on unshared locations, even if these are accessed within the scope of a transaction. Lack of identification of such instructions introduces a large performance overhead. The problem of identifying if an instruction is accessing a location which is shared, even if these locations are declared in advance, is a dynamic runtime problem, i.e. not solvable effectively through the use of a compiler. In the spirit of the new trend towards hardware assisted STMs (HASTMs), we show how one can add a simple hardware element, the Virtual Memory Filter (VMF), that provides dynamic identification and execution of STM functions on transactionally shared locations. The VMF will provide STMs with the simplicity of HTMs: atomic{} code can execute “as is.” Its introduction into commercial CPUs will eliminate the need to use a compiler to transactify user code, a benefit currently claimed only by full fledged HTM systems. Our preliminary empirical evidence shows that the VMF component has virtually no performance penalty.