CAR-STM: Scheduling-based Collision Avoidance and Reduction for Software Transactional Memory

Shlomi Dolev, Danny Hendler and Adi SuissaPODC 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-controlledapplication threads

Contention

Manager

ContentionDetection

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

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

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 processEnque 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. threadLock, condition var

Dispatcher / TQ-thread synchronization

TQ thread

Core #i

Transaction queue #i

Dispatcher

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

Ta

Transaction

queue #1

TQ thread

Core #1

PSCM

Tb

Transaction

queue #k

TQ thread

Core #k

Tc

Td Te

Transactions a and b collide, b is older

1

PSCM

Transaction queue #1

TQ thread

Core #1

PSCM

Tb

Transaction queue #k

TQ thread

Core #k

TaTc

Td Te

Losing transaction and its subordinates are made subordinates of winning transaction

Ta Tc

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 categorieso Long / short traversalso Short operations o Structure modification operations

Metrics and workload types

Workload typeReadsWrites

Read dominated

90%10%

Read/Write60%40%

Write dominated

10%90%

MetricsOperation typesComments

Execution timeAll5 min + quiescence

Quiescence timeAll

ThroughputAll except long traversals

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 secondso Randomly select regiono Randomly select transaction lengtho Randomly select operationo 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-dependentSome future work directions

Robust scheduling Transaction-aware OS scheduling Better handling of page faults, local data access,…