CAR-STM: Scheduling-based Collision Avoidance and Reduction for Software Transactional Memory Shlomi Dolev, Danny Hendler and Adi Suissa PODC 2008

CAR-STM: rationale  “transaction ignorant” thread scheduling problematic  TM scheduler handles transactional threads  This permits: o Serializing contention management o Proactive collision avoidance

“Conventional” STM system high-level structure OS-scheduler-controlled application threads Contention Manager Contention Detection arbitrate proceed Abort/retry/Wait TM System

CAR-STM's distinctive features Proactive Collision avoidance Proactively assign transaction thread to core with “most conflicting’’ transactions based on application-provided information Serializing contention management Serialize the execution of colliding transactions

Relying on (current) OS scheduling is problematic! 1)Introduces pseudo-parallelism 2)Hurts TM performance stability/predictability 3)Does not allow proactive collision avoidance and serializing CM. OS scheduling of transaction threads:

CAR-STM high-level architecture Transaction queue #1 TQ thread Transaction thread T-Info Core #1 Serializing contention mgr. Dispatcher Collision Avoider Core #k Transaction queue #k

TQ-Entry Structure Transaction queue #1 TQ thread Transaction thread T-Info Core #1 Serializing contention mgr. Dispatcher Collision Avoider Core #k Transaction queue #k wrapper method Transaction data T-Info Trans. thread Lock, condition var

Transaction dispatching process Call Dispatcher with an optional T-Info pointer argument 1 Call app-specific conflict probability method 3 Dispatcher calls Collision Avoider 2 Enque transaction in most-conflicting queue. Put thread to sleep, notify TQ thread. 4 4

Transaction execution TQ thread Core #i Transaction queue #i wrapper method Transaction data T-Info Trans. thread Lock, condition var TQ thread executes transaction 1 TQ thread wakes-up transaction thread 2 TQ thread dequeues entry 3

Dispatcher / TQ-thread synchronization TQ thread Core #i Transaction queue #i Dispatcher When TQ is emptied, TQ thread goes to sleep 1 When dispatcher adds a transaction, it wakes-up TQ thread 2

Serializing Contention Managers  When two transactions collide, fail the newer transaction and move it to the TQ of the older  Fast elimination of live-lock scenarios  Two SCMs implemented o Basic (BSCM) – move failed transaction to end of the other transactions' TQ o Permanent (PSCM) – Make the failed transaction a subordinate-transaction of the other transaction

PSCM TaTa Transaction queue #1 TQ thread Core #1 PSCM TbTb Transaction queue #k TQ thread Core #k TcTc TdTd TeTe Transactions a and b collide, b is older 1

PSCM Transaction queue #1 TQ thread Core #1 PSCM TbTb Transaction queue #k TQ thread Core #k TaTa TcTc TdTd TeTe Losing transaction and its subordinates are made subordinates of winning transaction TaTa TcTc

Experimental evaluation  Incorporated CAR-STM within RSTM  Tested on an 8-way 4 x XEON-7110M server  Serializing CM tests: Workloads generated by STMBEench7 [Guerraoui, Kapalka, Vitek, '07]  Proactive collision avoidance tested on synthetic app

STMBench7  A benchmark for STM implementations  Generates realistic workloads representative of complex, object-oriented applications  Workloads composed of 45 operation types on a shared data structure  Operation categories o Long / short traversals o Short operations o Structure modification operations

Metrics and workload types WritesReadsWorkload type 10%90%Read dominated 40%60%Read/Write 90%10%Write dominated CommentsOperation typesMetrics 5 min + quiescenceAllExecution time AllQuiescence time All except long traversalsThroughput

Execution time: R/W dominated workloads Speed-up of between 1.7 and 36 Reduction of standard deviation by factor of up to 40

Execution time: read dominated workloads

Execution time: Write dominated workloads

Quiescence time: a measure of live-lock Speed-up of between 11 and 118

Throughput: write dominated workloads Throughput increase of up to 15.7

Experimental evaluation: proactive collision avoidance  RegionedArray (RA) synthetic app (read, write, delete)  Each thread runs for 20 seconds o Randomly select region o Randomly select transaction length o Randomly select operation o Transaction repeatedly applies operation to randomly-selected region item Transactional memory Dagstuhl, June 08

Experimental results Transactional memory Dagstuhl, June 08

Most relevant prior art  [Yoo, Lee, 2008]: Adaptive transaction scheduling for TM systems  [Bai, Shen, Zhang, Scherer, Ding, Scott]: A key- based adaptive TM executor

Conclusions  Transactions-ignorant scheduling is problematic  Serializing contention management eliminates live-lock STM behavior  Proactive Collision avoidance contribution application-dependent Some future work directions  Robust scheduling  Transaction-aware OS scheduling  Better handling of page faults, local data access,…