Software Transactional Memory for Dynamic-Sized Data Structures(DSTM – Dynamic STM)

Maurice Herlihy,Victor Luchangco,Mark Moir,William N. Scherer III

PODC 2003

Presentation prepared by Adi Suissa for TM seminar, fall 2008/9

Overview

•Short recap and DSTM contributions•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

The computation modelStarting

transaction

Read-Transactional(o1)Write-Transactional(o2)

Read(o3)Write(o4)

Commit-Transaction

The computation model

•Committing a transaction can have two outcomes:▫Success: the transaction’s operations take

effect▫Failure: the operations are discarded

•Library Implemented in Java and in C++

Comparison with the Shavit-Toutitou TM

•Only static memory – need to declare the memory that can be transactioned statically▫We want the ability to create transactional

objects dynamically•Only static transactions – transactions need

to declare which addresses they are going to access before the transaction begins▫We want to let transactions determine which

object to access based on information of objects read inside a transaction

•Obstruction-free (rather than non-blocking)

Overview

•Short recap and what’s new?•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

Threads

•A thread that executes transactions must inherit from TMThread

•Each thread can run a single transaction at a time

class TMThread : Thread {void beginTransaction();bool commitTransaction();void abortTransaction();

}

Objects (1)

•All TM objects must implement the TMCloneable interface:

•This method clones the object, but programmers don’t need to handle synchronization issues

inteface TMCloneable {Object clone();

}

Objects (2)

• In order to make an object transactional, need to wrap it

•TMObject is a container for regular Java objects

Object

TMObject

Opening an object

•Before using a TMObject in a transaction, it must be opened

•An object can either be opened for READ or WRITE (and read)

class TMObject {TMObject(Object obj);enum Mode {READ, WRITE};Object open(Mode mode);

}

Overview

•Short recap and what’s new?•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

An atomic counter (1)

•The counter has a single data member and two operations:

•The object is shared among some threads

class Counter : TMCloneable {int counterValue = 0;

void inc(); // increment the valueint value(); // returns the valueObject clone();

}

An atomic counter (2)

•When a thread wants to access the counter in a transaction, it must first open the object using the encapsulated version:

Counter counter = new Counter();TMObject tranCounter = new TMObject(counter);

((TMThread)Thread.currentThread).beginTransaction();…Counter counter = (Counter)tranCounter.open(WRITE);counter.inc();…((TMThread)Thread.currentThread).commitTransaction();

Returns true/false to

indicate commit status

Overview

•Short recap and what’s new?•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

DSTM implementation

•Transactional object structure:

start

TMObject

transactionnew object

old object

status

Data

Data

Locator

Current object version

•The current object version is determined by the status of the transaction that most recently opened the object in WRITE mode:▫committed: the new object is the current▫aborted: the old object is the current▫active: the old object is the current, and

the new is tentative•The actual version only changes when the

commit succeeds

Opening an object (1)

•Lets assume transaction A opens object o in WRITE mode.

•Let transaction B be the transaction that most recently opened o in WRITE mode.

•We need to distinguish between the following cases:▫B is committed▫B is aborted▫B is active

Opening an object (2) – B committed

start

o

transactionnew object

old object

committed

Data

DataB’s Locator

1 A creates a new Locator

transactionnew object

old object

A’s Locator

2 A clones the previous new object, and sets new

Data

clone

3 A sets old object to the previous new

active4 Use CAS in

order to replace locator

If CAS fails, restarts from the beginning

Opening an object (3) – B aborted

start

o

transactionnew object

old object

aborted

Data

DataB’s Locator

1 A creates a new Locator

transactionnew object

old object

A’s Locator

2 A clones the previous old object, and sets new

Data

clone

3 A sets old object to the previous old

active4 Use CAS in

order to replace locator

Opening an object (4) – B active•Problem: B is active and can either commit

or abort, so which version (old/new) should we use?

•Answer: A and B are conflicting transactions, that run at the same time

•Use Contention Manager to decide which should continue and which should abort

• If B needs to abort, try to change its status to aborted (using CAS)

Opening an object (5)

•Lets assume transaction A opens object o in READ mode▫Fetch the current version just as before▫Add the pair (o, v) to the readers list (read-

only table)

Committing a transaction

•The commit needs to do the following:1. Validate the transaction2. Change the transaction’s status from

active to committed (using CAS)

Validating transactions•What?▫Validate the objects read by the transaction

•Why?▫To make sure that the transaction observes

a consistent state•How?

1.For each pair (o, v) in the read-only table, verify that v is still the most recently committed version of o

2.Check that (status == active)

If the validation fails, throw an exception so the user will restart the transaction from

the beginning

Validation inconsistency

•Assume two threads A and B

• If B after A, then o1 = 2, o2 = 1;• If A after B, then o1 = 1, o2 = 2• If they run concurrently we can have o1 =

1, o2 = 1 which is illegal

Thread A1. x <- read(o1)2. w(o2, x + 1)

Thread B1. y <- read(o2)2. w(o1, y + 1)

Initially:o1 = 0o2 = 0

Conflicts

•Conflicts are detected when:▫A transaction first opens an object and finds

that it is open for modification by another transaction

▫When the transaction validates its read set (on opening an object or commit)

Overview

•Short recap and what’s new?•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

Ordered Integer List – IntSet (1)

Min 3 4 8 Max

6

Ordered Integer List – IntSet (2)

class List implements TMCloneable {int value;TMObject next;

List(int v) { value = v; }

public Object clone() {List newList = new List(value);newList.next = next;return newList;

}}

Should have been called Element

Ordered Integer List – IntSet (3)

class IntSet {TMObject first; // the list’s anchor

IntSet() {List firstList = new List

(Integer.MIN_VALUE);first = new TMObject(firstList);firstList.next = new TMObject(

new List(Integer.MAX_VALUE));}

}

Ordered Integer List – IntSet (4)class IntSet {

boolean insert(int v) {List newList = new List(v);TMObject newNode = new

TMObject(newList);TMThread thread = Thread.currentThread();while (true) {

thread.beginTransaction();boolean result = true;try {

…} catch (Denied d) {}if (thread.commitTransaction())

return result;}

}}

Ordered Integer List – IntSet (5)

try {List prevList = (List)this.first.open(WRITE);List currList = (List)prevList.next.open(WRITE);while (currList.value < v) {

prevList = currList;currList = (List)currList.next.open(WRITE);

}if (currList.value == v) {

result = false;} else {

result = true;newList.next = prevList.next;prevList.next = newNode;

}}

Overview

•Short recap and what’s new?•How to use DSTM?•Example•Diving into DSTM•Example 2• Improving performance

Single entrance

•What is the problem with the previous example?

•How can it be solved?▫Opening for READ on traversal▫Maybe something more sophisticated?

Releasing an object

•An object that was open for READ can be released

•What does it imply?▫Careful planning▫Can increase performance▫What happens if we open an object, release

it and open it again in the same transaction?

▫Can lead to validation problems