[ 1 ] Agenda Overview of transactional memory (now) Two talks on challenges of transactional memory Rebuttals/panel discussion

Transactional Memory Overview Colin Blundell, E Christopher Lewis, Milo Martin University of Pennsylvania {blundell, lewis,

[ 3 ] The Challenge of Multithreaded SW Goal: Parallelization Problem: Unrestricted concurrency  bugs Solution: Synchronization New problem: Synchronization Tension between performance and correctness

[ 4 ] Current Mechanism: Locks Locks: objects only one thread can hold at a time Organization: lock for each shared structure Usage: (block)  acquire  access  release Correctness issues Under-locking  data races Acquires in different orders  deadlock Performance issues Conservative serialization Overhead of acquiring Difficult to find right granularity Blocking Correctness, efficiency, simplicity: choose two

[ 5 ] Enter Transactions Code segments with three features: a)Atomicity b)Serialization only on conflicts c)Rollback support

[ 6 ] Transactions vs. Locks Lock issues: – Under-locking – Acquires in different orders – Blocking – Conservative serialization How transactions help: + Simpler interface + No ordering + Can cancel transactions + Serialization only on conflicts Locks  simplicity/performance tension Transactions  (potentially) simple and efficient

[ 7 ] Agenda Transactions: what all the hoopla’s about Research Proposals Usages Implementations Disclaimer 1: Covering only hardware support Disclaimer 2: Purely an overview

[ 8 ] Execution Strategy Four main components: 1.Logging/buffering 2.Conflict detection 3.Abort/rollback 4.Commit Many implementation approaches

[ 9 ] Transactional Memory (TM) Goal: lock-free data structures Approach: multi-location atomic sequences (transactions) Implementation: special caches + traditional CC protocol Buffering? Transactional cache Conflict detection? Cache interventions Rollback? Invalidate transactional cache Commit? Validate transactional cache Transactions limited Bounded size Can’t survive interrupts No I/O [Herlihy & Moss, ISCA ‘93]

[ 10 ] Transactional Lock Removal (TLR) Goal: Lock-based code that serializes only on conflicts Approach: Execute critical sections as transactions Dynamically identify acquire/release pairs Handle conflicts using timestamps (no locks) Implementation: Buffer in caches + coherence protocol Reverts to locks in rare cases I/O Resource exhaustion Interrupts [Rajwar & Goodman, ASPLOS ‘02]

[ 11 ] Transactional Coherence and Consistency (TCC) Goal (1): Transactions for synchronization and parallelization Goal (2): Eliminate traditional cache coherence protocols Approach: All code in transactions Communication via transactions’ write packets Cache coherence at transaction-level granularity Programming model features Primitives for speculative parallelization Unbounded transactions and I/O via “early commit” [Hammond+, ISCA ‘04 & ASPLOS ‘04]

[ 12 ] Unbounded Transactional Memory Goal: Transactions for general-purpose synchronization Approach: In-progress transactions part of architected state + Unbounded & persistent transactions – Implementation difficulties (only abstract design) Compromise: LTM (caches + overflow table in memory) + More practical implementation – Transactions can’t survive interrupts [Ananian+, HPCA ‘05]

[ 13 ] Recent Proposals Virtualized Transactional Memory (VTM) [Rajwar+, ISCA ‘05] Goal: persistent transactions with less overhead Approach: group transactions by process Implementation: buffer in cache + overflow table in virtual memory + various interesting optimizations Go to talk! Thread-Level Transactional Memory (TTM) [Moore+, TR ‘05]

[ 14 ] Summary Transactions: Promising approach to synchronization Simple interface + efficient implementation Uses: optimistic lock removal, lock-free data structures, general-purpose synchronization, parallelization, ?? Challenges Implementation Interface OS involvement I/O + rollback See next two talks

[ 15 ] References Ananian+. Unbounded Transactional Memory. (HPCA 2005) Hammond+. Transactional Coherence & Consistency. (ISCA 2004) Hammond+. Programming with Transactional Coherence & Consistency (TCC). (ASPLOS 2004) Herlihy & Moss. Transactional Memory: Architectural Support for Lock-Free Data Structures. (ISCA 1993) Moore+. Thread-Level Transactional Memory. (UW TR 2005) Rajwar & Goodman. Transactional Lock-Free Execution of Lock-Base Programs. (ASPLOS 2004) Rajwar+. Virtualizing Transactional Memory. (ISCA 2005)