Concurrent Programming

1. Concurrent Programming Introduction to Locks and Lock-free data structures

2. Agenda • Concurrency and Mutual Exclusion • Mutual Exclusion without hardware primitives • Mutual Exclusion using locks and critical sections • Lock-based Stack • Lock freedom • Reasoning about concurrency: • Linearizability • Disadvantages of lock based data structures • A lock free stack using CAS • The ABA problem in the stack we just implemented Fix • Other problems with CAS • We need better hardware primitives • Transactional memory

3. Mutual Exclusion • Mutual Exclusion : aims to avoid the simultaneous use of a common resource • Eg: Global Variables, Databases etc. • Solutions: • Software: • Peterson’s algorithm, Dekker’s algorithm, Bakery etc. • Hardware: • Atomic test and set, compare and set, LL/SC etc. www.themegallery.com

4. Using the hardware instruction Test and Set • Test and Set, here on, TS: • TS on a boolean variable flag • #atomic // The two lines below will be executed one after the other without interruption • If(flag == false) • flag = true; • #end atomic boollock = false; // shared lock variable // Process i Init i; while(true) { while (lock==false){ // entry protocol TS(lock)}; Critical secion # i; lock = false; // exit protocol //Remainder of code;}

5. Software solution: Peterson’s Algorithm • One of the purely software solutions to the mutual exclusion problem based on shared memory • Simple solution for two processes P0 and P1 that would like share the use of a single resource R • More rigorously, P1 shouldn’t have access to R when P0 is modifying/reading R and vice-versa. R P0 P1

6. Peterson’s Algorithm: Two processor version Requires one global intvariable (turn), and one boolvariable (flag) per process. The global variable is turneach processor has signal a variable flag flag[0] = true is processor P0’s signal that it wants to enter the critical section turn = 0 says that it is processor P0’s turn to enter the critical section Can be extended to N processors

7. How to think about • Consider you are in a hall way that is only wide enough for one person to walk. • However, you a see a guy walking in the opposite direction as you are. • Once you approach him, you have two options: • Be a gentleman and step to the side so that he may walk first, and you will continue after he passes ( Peterson’s algorithm) • Beat him up and walk over him (Critical section violation)

8. The algorithm in code // Process 0 init; while(true) { // entry protocol flag[0] = true; turn = 1; while (flag[1] && turn == 1) {}; critical section #0; // exit protocol flag[1] = false; //remainder code } // Process 1 init; while(true) { // entry protocol flag[1] = true; turn = 0; while (flag[0] && turn == 0) {}; critical section #1; // exit protocol flag[1] = false; //remainder code } // Shared variables bool flag[2] = {false, false}; int turn = 0;

9. Requirements for Peterson’s • Reads and writes have to atomic • No reordering of instructions or memory • In order processors sometime reorder memory accesses even if they don’t reorder memory accesses. • In that case one needs to use memory barrier instructions • Visibility: Any change to a variable has to take immediate effect so that everybody knows about. • Keyword volatile in Java www.themegallery.com

10. So why don’t people use Peterson’s? • Notice the while loop in the algorithm • If process 0 waits a lot of time to enter the critical section, it continually checks the flag and turn to see it can or not, while not doing any useful work • This is termed busy waiting, and locking mechanisms like Peterson’s have a major disadvantage in that regard. • Locks that employ continuous checking mechanism for a flag are called Spin-Locks. • Spin locks are good when the you know that the wait is not long enough. while (flag[1] && turn == 1) {};

11. Properties of Peterson’s algorithm • Mutual Exclusion • Absense of Livelocks and Deadlocks: • A live lock is similar to a dead lock but the states of competing processes continually change their state but neither makes any progress. • Eventual Entry: is guaranteed even if scheduling policy is only weakly fair. • A weakly fair scheduling policy guarantees that if a process requests to enter its critical section (and does not withdraw the request), the process will eventually enter its critical section.

12. Comparison with Test and Set

13. Putting it all together: a lock based Stack • Stack: A list or an array based data structure that enforces last-in-first-out ordering of elements • Operations • Void Push(T data) : pushes the variable data on to the stack • T Pop() : removes the last item that was pushed on to a stack. Throws a stackEmptyException if the stack is empty • Int Size() : returns the size of the stack • All operations are synchronized using one common lock object. www.themegallery.com

14. Code : Java Class Stack<T> { ArrayList<T> _container = new ArrayList<T>(); RentrantLock _lock = new ReentrantLock(); public void push(T data){ _lock.lock(); _container.add(data); _lock.unlock();} public int size(){ intretVal; _lock.Lock(); retVal = _container.size(); _lock.unlock(); return retVal; } public T pop(){ _lock.lock(); if(_container.empty()) { _lock.unlock(); throw new Exception(“Stack Empty”);} T retVal _container.get(_container.size() – 1); _lock.unlock(); return retVal; }

15. Problems with locks • Stack is simple enough. There is only one lock. The overhead isn’t that much. But there are data structures that could have multiple locks • Problems with locking • Deadlock • Priority inversion • Convoying • Kill tolerant availability • Preemption tolerance • Overall performance

16. Problems with locking 2 • Priority inversion: • Assume two threads: • T1 with very low priority • T2 with very high priority • Both need to access a shared resource R but T2 holds the lock to R • T2 takes longer to complete the operation leaving the higher priority thread waiting, hence by extension T1 has achieved a lower priority • Possible solution Priority inheritance

17. Problems with Locking 3 • Deadlock: Processes can’t proceed because each of them is waiting for the other release a needed resource. • Scenario: • There are two locks A and B • Process 1 needs A and B in that order to safely execute • Process 2 needs B and A in that order to safely execute • Process 1 acquires A and Process two acquires B • Now Process 1 is waiting for Process 2 to release B and Process 2 is waiting for process 1 to release A

18. Problems with Locking 4 • Convoying, all the processes need a lock A to proceed however, a lower priority process acquires Ait first. Then all the other processes slow down to the speed of the lower priority process. • Think of a freeway: • You are driving an Aston Martin but you are stuck behind a beat up old pick truck that is moving very slow and there is no way to overtake him.

19. Problems with Locking 5 • Kill tolerance • What happens when a process holding a lock is killed? • Everybody else waiting for the lock may not ended up getting it and would wait forever. • ‘Async-signal safety’ • Signal handlers can’t use lock-based primitives • Why? • Suppose a thread receives a signal while holding a user level lock in the memory allocator • Signal handler executes, calls malloc, wants the lock

20. Problems with Locking 6 • Overall performance • Arguable • Efficient lock-based algorithms exist • Constant struggle between simplicity and efficiency • Example. thread-safe linked list with lots of nodes • Lock the whole list for every operation? • Reader/writer locks? • Allow locking individual elements of the list?

21. A Possible solution Lock-free Programming

22. Lock-free data structures • A data structure wherein there are no explicit locks used for achieving synchronization between multiple threads, and the progress of one thread doesn’t block/impede the progress of another. • Doesn’t imply starvation freedom ( Meaning one thread could potentially wait forever). But nobody starves in practice • Advantages: • You don’t run into all the that you would problems with using locks • Disadvantages: To be discussed later

23. Lock-free Programming • Think in terms of Algorithms + Data Structure = Program • Thread safe access to shared data without the use of locks, mutexes etc. • Possible but not practical/feasible in the absence of hardware support • So what do we need? • A compare and set primitive from the hardware guys, abbreviated CAS(To be discussed in the next slide) • Interesting TidBit: • Lots of music sharing and streaming applications use lock-free data structures • PortAudio, PortMidi, and SuperColliderPortAudio

24. Lock-free Programming • Compare and Set primitive • booleancas( int * valueToChange, int * valueToSet To, int * ValueToCompareTo) • Sematics: The pseudocode below executes atomically without interruption • If( valueToChange == valueToCompareTo){ valueToChange= valueToSetTo; return true; } else { return false; } • This function is exposed in Java through the atomic namespace, in C++ depending on the OS and architecture, you find libraries • CASis all you need for lock-free queues, stacks, linked-lists, and sets.

25. Trick to building lock-free data structures • Limit the scope of changes to a single atomic variable • Stack : head • Queue: head or tail depending on enque or deque

26. A simple lock-free example • A lock free Stack • Adopted from Geoff Langdaleat CMU • Intended to illustrate the design of lock-free data structures and problems with lock-free synchronization • There is a primitive operation we need: • CAS or Compare and Set • Available on most modern machines • X86 assembly: xchg • PowerPC assembly: LL(load linked), SC (Store Conditional)

27. Lock-free Stack with Ints in C • A stack based on a singly linked list. Not particularly good design! • Now that we have the nodes let us proceed to meat of the stack • structNodeEle { • int data; • Node *next; • }; • typedefNodeEle Node; • Node* head; // The head of the list

28. Lock-free Stack Push void push(int t) { Node* node = malloc(sizeof(Node)); node->data = t; do { node->next = head; } while (!cas(&head, node, node->next)); } Let us see how this works!

29. Push in Action • Currently Head points to the Node containing data 6 Head 10 6

30. Push in Action • Two threads T1 and T2 comes along wanting to push 7 and 8 respectively, by calling the push function Head T2 T1 push(8); push(7); 10 6

31. Push in Action • Two new node structs on the heap will be created on the heap in parallel after the execution of the code shown Head Node* node = malloc(sizeof(Node)); node->data = 8; T2 T1 Node* node = malloc(sizeof(Node)); node->data = 7; 10 6

32. Push in Action • The above code means set the newly created Nodes next to head, if the head is still points to 6 then change head pointer to point to the new Node • Both of them try to execute this portion of the code on their respective threads. But only one will succeed. 7 8 Head T1 T2 do { node->next = head; } while (!cas(&head, node, node->next)); 10 6

33. Push in Action • Let us Assume T1 Succeeds, therefore T1 exits out of the while and consequently the push() • T2’s cas failed why? Hint: Look at the picture. • T2 has no choice but to try again 7 Head 8 T1 T2 do { node->next = head; } while (!cas(&head, node, node->next)); 10 6

34. Push in Action • Assume T2 Succeeds this time because no one else trying to push 7 8 Head T1 T2 10 6

35. Pop() bool pop(int& t) { Node* current = head; while(current) { if(cas(&head, current->next, current)) { t = current->data; // problem? return true; } current = head; } return false; } There is something wrong this code. It is very subtle. Can you figure it out? Most of the time this piece of code will work.

36. It is called the ABA problem • While a thread tries to modify A, what happens if A gets changed to B then back to A? • Malloc recycles addresses. It has to eventually. • Now Imagine this scenario. • Curly braces contain addresses for each node { 0x89} { 0x90} Head 10 6

37. ABA problem illustration Step 1 • Assume two threads T1 and T2. • T1 calls pop() to delete Node at 0x90 but before it has a change and CAS, there is a context switch and T1 goes to sleep. { 0x89} { 0x90} Head bool pop(int& t) { Node* current = del = head; while(current) { if(cas(&head, current->next, current)) { t = current->data; // problem? delete del; return true; } current = head; } return false; } 10 6 www.themegallery.com

38. ABA problem illustration Step 2 • The following happens while T1 is asleep { 0x89} { 0x90} Head 10 6 www.themegallery.com

39. ABA problem illustration Step 3 • The following happens while T1 is asleep • T2 calls Pop(), Node at 0x90 is deleted { 0x89} { 0x90} Head 10 6 www.themegallery.com

40. ABA problem illustration Step 4 • The following happens while T1 is asleep • T2 calls Pop(), Node at 0x90 is deleted • T2 calls Pop(), Node at 0x89 is deleted { 0x89} { 0x90} Head 10 6 www.themegallery.com

41. ABA problem illustration Step 5 • The following happens while T1 is asleep • T2 calls Pop(), Node at 0x90 is deleted • T2 calls Pop(), Node at 0x89 is deleted • T2 calls push(11) but malloc has recycled the memory 0x90 while allocating space for the new Node { 0x89} { 0x90} Head 10 11 www.themegallery.com

42. ABA problem illustration Step 6 • The following happens while T1 is asleep • T2 calls Pop(), Node at 0x90 is deleted • T2 calls Pop(), Node at 0x89 is deleted • T2 calls push(11) but malloc has recycled the memory 0x90 while allocating space for the new Node • T1 now wakes up and the CAS operation succeeds { 0x89} { 0x90} Head Head is now pointing to illegal memory!!!! 10 11 Replace 10 and 6 with B and A Now you know where the name (ABA) comes from www.themegallery.com

43. Solutions: • Double word compare and set. • One 32 bit word for the address • One 32 bit word for the update count which is incremented every time a node is updated • Compare and Set iff both of the above match • Java provides AtomicStampedReference • Use the lower address bits of the pointer (if the memory is 4/8 byte Aligned) to keep a counter to update • But the probability of a false positive is still greater than doublewordcompareandset because instead of 2^32 choices for the counter you have 2^2 or 2^3 choices for the counter

44. Disadvantages of lock-free data structures • Current hardware limits the amount of bits available in CAS operation to 32/64 bits. • Imagine the implementation of data structures like BST’s pose a problem • When you need to balance a tree you need update several nodes all at once. • Way to get around it • Transactional memory based systems

45. Language Support • C++ 09 (the new C++) : atomic_compare_exchange() • Current C++: pthreads library • GCC: • type __sync_val_compare_and_swap (type *ptr, typeoldval, typenewval) • More info: http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html • Java • Package: java.util.concurrent.atomic.* • AtomicInteger, AtomicBooleanetc.: Atomic access to a single intboolean etc • AtomicStampedReference: Associates an int with a reference • AtomicMarkableReference: Associates a boolean with a reference • Your own CAS: • Write an inline assembly function that uses XCHG or LL/SC depending on the hardware platform

46. Performance of Lock-based vs. Lock-free under moderate contention

47. Performance of Lock-based vs. Lock-free under very high contention (almost unrealistic)

48. Ensuring correctness of concurrent objects • To ensure two properties of concurrent objects (eg: FIFO queues) • Safety: Object behavior is correct as per the specification • Behavior a FIFO queue: • If two enques x and y happens in parallel(assume queue is initially empty) then the next deque should only return either x or y not z • If enques y and z happened one after other in real time, then deque() should return y first and z second • Overall progress: Conditions under which at least one thread will progress

49. Ensuring correctness in concurrent implementations • Linearizability: • Each method call(enq and deq in the case of queue) should appear to take place instantaneously sometime between the start and the end of the method call • Meaning no other thread can see the change to the data structure in a step by step fashion • In English: If the concurrent execution can be mapped to a valid(meaning correct) sequential execution on the object, then we assume that it is correct. • Moreover, this can be used as an intuitive way to reason about concurrent objects. • You already know it because you use it unknowingly • Think of shared single lock FIFO queue

50. Linearizability: Intuitively • Consider the deq() method for a queue • Uses a single shared lock for mutual exclusion publicT deq() throwsEmptyException { lock.lock(); try { if (tail == head) throw new EmptyException(); T x = items[head % items.length]; head++; return x; } finally { lock.unlock(); } } All modifications of queue are done mutually exclusive. Therefore essentially happens in sequence.