UnreadTVar: Extending Haskell Software Transactional Memory for Performance

As new trends in computer architecture yield towards shared-memory chip multiprocessors (CMP), the rules for programming these machines are significantly changing. In the search for alternatives to lock-based concurrency protocols, Software Transactional Memory extensions to Haskell have provided an easy-to-use lock-free abstraction mechanism for concurrent programming, using atomically composed blocks operating on transactional variables. However, as in the case for linked structures, the composition of these atomic blocks require a bit more attention, as atomicity might attempt to “look after ” more than what is needed and cause an overall decrease in performance. To prevent this situation, we have extended the Transactional Memory module of The Glasgow Haskell Compiler 6.6 to support a construct that we term unreadTVar, which is introduced to improve execution time and memory usage when traversing transactional linked structures, while requiring a little more care from the programmer. The experimentations done with linked lists show that the proposed approach leads to a substantial performance increase for large transactional linked structures.