1. Maurice Herlihy, Victor Luchangco, Mark Moir, William N. Scherer III
Software Transactional Memory for Dynamic-Sized Data Structures (DSTM – Dynamic STM)
Maurice Herlihy,
Victor Luchangco,
Mark Moir,
William N. Scherer III
PODC
Presentation prepared by Adi Suissa for TM seminar, fall 2008/9

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

3. The computation model Starting transaction Read-Transactional(o1)
Write-Transactional(o2)
Read(o3)
Write(o4)
Commit-Transaction

4. 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++

5. 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)

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

7. 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(); }

8. 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(); }

9. Objects (2) In order to make an object transactional, need to wrap it
TMObject is a container for regular Java objects
TMObject
Object

10. 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); }

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

12. 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 value
int value(); // returns the value
Object clone(); }

13. Returns true/false to indicate commit status
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

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

15. DSTM implementation Transactional object structure: status transaction
new object
start
Data
old object
TMObject
Locator
Data

16. 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

17. 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

18. Opening an object (2) – B committed
transaction
Data
new object
start
old object
If CAS fails, restarts from the beginning o
Data
B’s Locator
clone 4
Use CAS in order to replace locator
transaction
active
new object
Data
old object
A’s Locator 1 3
A creates a new Locator
A sets old object to the previous new 2
A clones the previous new object, and sets new

19. Opening an object (3) – B aborted
transaction
Data
new object
start
old object o
Data
B’s Locator 4
Use CAS in order to replace locator
transaction
active
clone
new object
Data
old object
A’s Locator 1 3
A creates a new Locator
A sets old object to the previous old 2
A clones the previous old object, and sets new

20. 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)

21. 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)

22. Committing a transaction
The commit needs to do the following:
Validate the transaction
Change the transaction’s status from active to committed (using CAS)

23. Validating transactions
What?
Validate the objects read by the transaction
Why?
To make sure that the transaction observes a consistent state
How?
For each pair (o, v) in the read-only table, verify that v is still the most recently committed version of o
Check that (status == active)
If the validation fails, throw an exception so the user will restart the transaction from the beginning

24. 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 A
1. x <- read(o1)
2. w(o2, x + 1)
Thread B
1. y <- read(o2)
2. w(o1, y + 1)
Initially:
o1 = 0
o2 = 0

25. 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)

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

27. Ordered Integer List – IntSet (1)
Min 3 4 8
Max 6

28. 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

29. 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)); }

30. 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; }

31. 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;

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

33. Single entrance What is the problem with the previous example?
How can it be solved?
Opening for READ on traversal
Maybe something more sophisticated?

34. 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

35. Questions?