Microsoft Research Faculty Summit 2007

Panacea or Pandora’s Box? Software Transactional Memory Panacea or Pandora’s Box? Christos Kozyrakis Assistant Professor of EE & CS Stanford University

My Message In a Nutshell Transactional Memory (TM) Atomic access to shared state User declares parallelism  system manages The bad news Still in relatively early R&D stages TM cannot solve all problems around parallelism The good news TM is an very powerful tool Its uses go beyond concurrency management Needed: systems  users & apps  feedback

Programming with TM Integration with HL languages and compilers Java/C# (PLDI’06, PPoPP’07), OpenMP (PACT’07), … Declarative approach to sharing data Rich features beyond atomic{} Iterators, variable types, retry, abstract locks, … Speculative parallelization Still missing: Truly HL parallel model(s) E.g., DBs use atomic{} but program SQL Deal with finding concurrency, locality, coordination, and scalability in addition to atomicity

TM And I/O Can have some I/O model within TM but… Special libraries, restricted APIs, … Better idea: System-level transactions TM is one of the many system resources Use transactional mechanisms with other resources FS  LFS, Net  message queues, DBMS Coordinate transactions across multiple resources Easier-to-use & flexible model Can this ever work? Look at IBM’s Quicksilver project from the 80s

Beyond Concurrency When you build TM, you build Atomicity, consistency, and isolation Mechanisms to monitor memory accesses Great building blocks for Debugging (deterministic replay, watchpoints, parallel bookmarks) Profiling & tuning (contention & locality monitoring) Security (isolated execution, canaries) Fault-tolerance (undo on error, checkpointing) … These can be killer apps for TM Equally important for programmability

Can TM use HW Support? Yes HW reduces TM overheads for common case Don’t forget that performance is **the** motivation for parallel programming Low overhead is particularly important when TM is used extensively for non-concurrency purposes But HW provides basic mechanisms, SW sets policies E.g., SigTM design at ISCA’07 HW should have flexible semantics (see ISCA’06) TM apps should work even without HW support

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