1. Transactional Memory Overview Olatunji Ruwase 15-740 Fall 2007 Oct 4 2007

2. Future PCs are Multicore

3. Parallel s/w for performance Locks Deadlocks Priority Inversion Buggy Convoy Effect Limited concurrency

4. Transactions Borrowed from Databases Definition : A transaction is a finite sequence of machine instructions executed by a single process, that satisfies the following properties Atomicity Serializability Herlihy and Moss. “Transactional Memory: Architectural Support for Lock-free Data Structures, ISCA’93

5. How transactions work Make private copy of shared data Make updates on private copy If shared data is unchanged Update shared data with private copy Else conflict has occurred Discard private copy and repeat transaction

6. Requirements for supporting transactions Buffering Conflict detection Abort/Rollback Commit

7. Transactional Memory Herlihy and Moss, ISCA ’93 ISA changes LT,LTX, ST, COMMIT, VALIDATE, ABORT Fully associative transactional cache Per processor Contents are exclusive of regular cache Cache coherence protocol changes Transactional cache line states Transactional bus messages Snoopy bus based implementation

8. TM support for transactions BufferingTransactional cache Conflict detectionCache coherence protocol Abort/RecoveryInvalidate transactional cache line CommitValidate transactional cache line

9. Transactional Cache Fully set associative cache Each cache line can be in only one of transactional or regular cache Holds transactional writes Transactional writes are hidden from other processors and memory Makes updated lines available for snooping on COMMIT Invalidate updated line on ABORT

10. Herlihy and Moss, ISCA ‘93 M S S XCommit XAbort CacheTransaction Cache CPU Memory

11. Sample Counter code

12. Exposing more concurrency Doubly linked list implementation of queue Head, Tail pointers If queue not empty Only head pointer is used for dequeuing Only tail pointer is used for enqueuing Concurrent enqueuing/dequeuing Possible in TM Not possible with locks

13. Challenges of TM Long transactions I/O Nested transactions Interrupts

14. Other TM Ideas Speculative Lock Elision Software Transactional Memory Requires no hardware changes Allows composition of transactions

15. Speculative Lock Elision Ravi and Goodman, MICRO ‘01 Speculatively remove lock acquire and removal instructions Microarchitectural changes No changes to cache systems No changes to ISA Can work with existing lock based code

16. SLE example

17. Credits for slides Bryant Lee (CMU) Transactional Memory: Principles and Current Research Colin Blundell (UPenn) Transaction Memory Overview Kevin Moore (UW) Thread-Level Transactional Memory

18. Compare TM and TLS TM is optimistic synchronization TLS is optimistic parallelization Any other similarities or differences

19. Questions/Discussions