Why STMs Need Why STMs Need CompilersCompilers(a war story)(a war story)

Maurice HerlihyBrown University

STMSTM

Interested in managed languagesJava, C#Not C, C++

Synchronization based on objects, not addresses

Strong isolation between transactional & non-transactional access

STM Libraries: DSTM 2003STM Libraries: DSTM 2003

Transactional “wrapper” for objects

Cumbersome & error-prone

// wrapper for node objectTMObject<RBNode> xnode = …// explicit openRBNode node = xnode.openRead();// next field is wrapperTMObject<RBNode> next = node.next;

STM Libraries: SXM 2005STM Libraries: SXM 2005

Mark data types as atomicUse reflection & run-time code generation

Not efficient (no global optimizations)

[Atomic] // Atomic Red-Black Tree nodepublic class RBNode { public int value; public Color color; public bool marked; public RBNode parent, left, right;

public RBNode() { value = 0; color = Color.RED; parent = null; left = null; right = null; }}

Current workCurrent work

Mark data types as atomicCompiler does the rest…

[Atomic] // Atomic Red-Black Tree nodepublic class RBNode { public int value; public Color color; public bool marked; public RBNode parent, left, right;

public RBNode() { value = 0; color = Color.RED; parent = null; left = null; right = null; }

// other methods}

OptimizationsOptimizations

Whole object dataflow analysiscall tree for non-virtual methods Pass context from method to method

Every method is analyzed for:Local objects (not shared across transactions)Read only or read/write objects

Promotes openReads to openWrites for RW objects

Early opens for objects used across basic blocks

Partial Redundancy Elimination (PRE)

Optimizations (cont.)Optimizations (cont.)

First vs. subsequent read/writesInline subsequent reads/writes with fast-path code sequenceSubsequent reads/writes are lock-free, even in the STM mode that uses short locks

Progress ConditionsProgress Conditions

Short-lived locksBlockingEfficient library implementation

Obstruction-freeNo locks, non-blockingInefficient library implemenetation

After compiler optimizations,obstruction-free performs as well as lock-based

Performance ComparisonPerformance ComparisonBlocking Mode

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

List RBTree SkipList HashTable Buffer

Benchmark

Op

s/s

ec

SXM

Library Opts

Compiler

Const + Local

RWPromo

Subsequent

PreOpen

Performance ComparisonPerformance Comparison

Obstruction-Free Mode

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

List RBTree SkipList HashTable Buffer

Benchmark

Op

s/s

ec

SXM

Library Opts

Compiler

Const + Local

RWPromo

Subsequent

PreOpen

Moral of the storyMoral of the story

Libraries are eitherLow-overhead but hard to useInefficient but easy to use

Compiler supportCombines best of both worldsNon-local optimization