transactional memory lecturer: danny hendler. speeding up uni-processors is harder and harder ...

Transactional Memory

Lecturer: Danny Hendler

Speeding up uni-processors is harder and harder

Intel, Sun (RIP), AMD, IBM now focusing on “multi-core” architectures

Already, most computers are multiprocessors

How can we write correct and

efficient algorithms for

multiprocessors?

The Future of Computing

A fundamental problem of thread-level parallelism

. .Account[i] = Account[i]-X;Account[j] = Account[j]+X; .

. .

. .Account[i] = Account[i]-X;Account[j] = Account[j]+X; .

. .

Thread A Thread B

But what if execution is concurrent?

Must avoid race

conditions

Inter-thread synch. alternatives

What is a transaction?

• A transaction is a sequence of memory reads and writes, executed by a single thread, that either commits or aborts

• If a transaction commits, all the reads and writes appear to have executed atomically

• If a transaction aborts, none of its stores take effect

• Transaction operations aren't visible until they commit (if they do)

Transactions properties:A transaction satisfies the following key

property:

• Atomicity: Each transaction either commits (its changes seem to take effect atomically) or aborts (its changes have no effect).

• Serializability: all committed transactions issue the same operations and receive the same responses as in some sequential history consisting only of committed transactions.• Some work considers weaker or stronger requirements

• Isolation: Transaction writes are not visible outside the transaction until it commits

Transactional Memory Goals

• A new multiprocessor architecture• The goal: Implementing nonblocking

synchronization that is– efficient– easy to use compared with conventional techniques

based on mutual exclusion

• Implemented by hardware support (such as straightforward extensions to multiprocessor cache-coherence protocols) and / or by software mechanisms

A Usage Example

Locks:

Lock(L[i]); Lock(L[j]); Account[i] = Account[i] –

X; Account[j] = Account[j] +

X; Unlock(L[j]); Unlock(L[i]);

Transactional Memory:

atomic { Account[i] = Account[i] –

X; Account[j] = Account[j] +

X; };

Account[i] = Account[i]-X;Account[j] = Account[j]+X;

• Transactions execute in commit order

ld 0xdddd...st 0xbeef

Transaction ATime

ld 0xbeef

Transaction C

ld 0xbeef

Re-execute Re-execute with new datawith new data

Commit

ld 0xdddd...ld 0xbbbb

Transaction B

Commit Violation!Violation!

0xbeef0xbeef

Taken from a presentation by Royi Maimon & Merav Havuv, prepared for a seminar given by Prof. Yehuda Afek.

Transactions interaction

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

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

PODC 2003

Prepared by Adi Suissa

Motivation

•Transactional Memory – simplifies parallel programming

•STM – Software based TM▫Usually simpler than Hardware based TM▫Can handle situations where HTM fails

•However:▫It is immature (supports static data sets and

static transactions)▫It is complicated

Overview

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

Transactions

•Transaction – a sequence of steps executed by a single thread

•Transactions are atomic: each transaction either commits (it takes effect) or aborts (its effects are discarded)

•Transactions are linearizable: they appear to take effect in a one-at-a-time order

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

• Implemented in Java and in C++

Previous STM designs

•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

and this is why it is called Dynamic Software

Transactional Memory

Overview


Threads

•A thread that executes transactions must be inherited from TMThread

•Each thread can run a single transaction at a time

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

}

Don’t forget the run() method

Objects (1)

•All transactinal objects must implement the TMCloneable interface:

•This method clones the object, but clone implementors 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


An atomic counter (1)

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

•The object is shared by multiple 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


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 a

commit is successful

Opening an object (1)

•Let's assume transaction A tries to open 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


old object

committed

Data

DataB’s Locator

1 A creates a new Locator


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, A restarts from the beginning

Opening an object (3) – B aborted

start

o


old object

aborted

Data

DataB’s Locator

1 A creates a new Locator


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 (only) 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


Ordered Integer List – IntSet (1)

Min 3 4 8 Max

6


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;

}}


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;}

}}


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


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

Overview


Non-Blocking Algorithms

•A family of algorithms on a shared data•Each sub-family satisfies different

progress guarantees•Usually, there is a correlation between the

progress guarantee strength and the complexity of the algorithm

Wait-Free algorithms

•An algorithm is wait-free if every operation has a bound on the number of steps it will take before completing

No Starvation

Lock-Free algorithms

•An algorithm is lock-free if every step taken achieves global progress

•Even if n-1 processes fail (while doing operations on the shared memory), the last processor can still complete its operation

•Example: Shavit & Touitou’s STM implementation

Obstruction-Free algorithms

•An algorithm is obstruction-free if at any point, a single thread executed in isolation for a bounded number of steps will complete its operation

•Doesn’t avoid live-locks•Example: DSTM implementation•What is it good for?

Contention Manager (CM)

•The contention manager arbitrates between two conflicting transactions

•Given two (conflicting) transactions TA, TB, then CM(TA, TB):1. Decides who wins2. Decides what the loser should do

(abort/wait/retry)•Conflicts policy

transactional memory lecturer: danny hendler. speeding up uni-processors is harder and harder ...

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