A Novel Concurrent Validation Scheme for Hardware Transactional Memory

Transactional memory is a lock-free parallel programming model, which aims at replacing conventional lock-based threaded programming techniques, currently used by multi-core systems. These techniques are difficult to implement and impose unnecessary overheads caused by conservative programming practices. In this thesis, the scalability potential of a transactional memory system, called TMFab, was explored for different numbers of processors and it was concluded that for more than 4 processors the system presents reduced scalability, due to an increase in the validation overhead. In response to this observation, a novel validation scheme was proposed which reduces this overhead, first by allowing multiple transactions to perform their validations and commit operations concurrently, and second by removing the need for broadcasting messages between the active transactions. A distributed shared memory scheme was used to increase the validation and memory access throughput, as well as allow for transactions to commit concurrently on different memory partitions. The two architectures were compared by means of SystemC simulation, and a maximum of 2.5x validation speedup was observed for the modified design, together with a 2.7x reduction in memory access latency. In total, the modified design achieved a maximum execution speedup of 30% over the original, for the benchmarks that were used. Furthermore, the modified system guarantees sequential consistency even in corner case scenarios.