1. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Rich Transactions on Reasonable Hardware J. Eliot B. Moss Univ. of Massachusetts, Amherst moss@cs.umass.edu

2. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Language/Virtual Machine Challenges Common uses of TM simple for programmer Semantics simple, clean, useable Realize potential programming benefits Advanced uses possible and clean Overcome hardware limitations Size of transactions Semantics of what’s built in

3. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science The Simplest Case Setting: Java (can explore other platforms) Credit to Harris and Fraser for simple form: atomic { statements } Executes statements: Either entirely or not at all Repeated until succeeds: exactly-once effect A single state transition (as seen by others) Provides a simple model: ACI of ACID

4. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Mapping Simple Form to Hardware Transaction = read & write set of cache lines Conflict if write line touched by other txn Abort = drop set of records, buffered writes Commit = drop records, install writes Issue: Txns with large sets Need graceful overflow to memory Perhaps even to virtual memory (disk)

5. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Semantics of Nesting Actions Harris & Fraser: Aggregate into one big set This one big txn commits only at top level Closed Nesting: Sub-txn can abort without top aborting Sub-txn commits “into” top’s sets Retry high-conflict part w/o aborting all Good for libraries, composing modules Allows alternate strategies, optional parts,... m-of-n replica commit,...

6. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Semantics of Nesting Actions (2) No additional syntax, richer semantics Hardware support needed: Tag lines with txn id (SMTs need anyway) Multiple copies of same line; different ids Txn id analogous to address space ID Can get by with a handful of bits Add parent/child txn table More complex txn conflict detect logic Txn relationships can be cached (like TLB)

7. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Predicated (Conditional) Actions Harris and Fraser construct: atomic (predicate) { stmts } stmts executed only when predicate is true Evaluate predicate (as part of an action) If true, continue to evaluate stmts predicate and stmts part of same action If false, “wait until predicate is true” In hardware, set to lowest priority, then... Wait until read/write set conflict and retry

8. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Predicated Actions: Hardware Need “Low priority” notion, so waiter aborted Giving up after a while, controls on how long Possibly maintaining state for waiting thread No explicit notify: conflict is the indicator If predicate tends to be cheap, just retry Kicking up to s/w for more explicit handling

9. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Beyond Closed Nesting Closed nesting only builds largers sets Consider modifying a cache or table New/deleted entries touched: ok, since want to block access to modifications Things along path in data structure: creates often undesirable conflicts Need: Distinguish physical data structure from its logical contents

10. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Beyond Closed Nesting: Open Nesting Open nesting: Open nested action truly commits changes Leaves explicit access control information Access control managed by software Abort of enclosing txn requires undo Retry of enclosing txn requires redo

11. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Open Nesting Syntax/Design Possible syntax: open atomic { stmts } undo { u } redo { r } Implementation strategy: Language run-time builds undo/redo lists When needed, they are run as txns Need access to txn ids, conflict tests, etc. Transactions become explicit, can be named and manipulated

12. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Some Applications of Open Nesting Shared table structures (already noted) May help support GC: Open nested commit updates (some) data held by enclosing atomic blocks: supports GC work done by executing thread Admittedly idea still rough... GC from other threads may need “reflective” ability to see/update transactional data of application threads...

13. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Other Desired Features atend actions: happen at end of top-level May help with issue of I/O Connection to OS supported durability Specially designated pages + commit call Lock coupling: “release locks” on parts of data structures walked but not needed E.g.: searching list linearly to remove item

14. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Other Desired Features (2) Checkpointing and rollback to checkpoints Concurrency: spawning parallel sub-txns Transaction per loop iteration Control on ordering relationships H/W:OS desire: cheap thread spawning Implication: multiple threads, each working within the same transaction hierarchy Again, explicit txn ids make this easier

15. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Non-Normal Outcomes Harris and Fraser exception semantics: Throwing is a transaction success Usual Java semantics Claim: More useful to undo visible effects Abort writes to previously existing objects Retain objects created in failing txn Allows return of reason for failure Example: Denied credit

16. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Non-Normal Outcomes (2) Example syntax: fail with new Exc(...); Challenge: keeping some state, tossing rest May be able to implement this with combination of open nesting and run-time system support: In open nested action, create copy of newly allocated reachable objects and commit the open nested txn