Threads

1. Threads Tutorial #7 CPSC 261

2. A thread is a virtual processor • Each thread is provided the illusion that it owns a core • Copy of the registers • It is running all the time • In fact, all of the threads share the hardware cores • The operating system rapidly switches the cores among the threads that want to run, providing this illusion that each thread owns a core

3. POSIX standard: pthreads • Threads are created via pthread_create() • A thread dies when it: • returns from the function given to pthread_create • or calls pthread_exit() • You can wait for a thread to complete via pthread_join()

4. Visualizing thread execution main thread pthread_create() child threads pthread_join()

5. PingPong.c void *p(void *arg) { long i; for (i = 0; i < LIMIT; ++i) { counter = counter + 1; } return 0; } void main(...) { pthread_create(&t1, NULL, &p, "ping”); pthread_create(&t2, NULL, &p, "pong”); pthread_join(t1, NULL); pthread_join(t2, NULL); }

6. Questions you might ask • What if t2 finishes before t1? • It will just wait as long as necessary for the main thread to join with it • How many threads can I create? • It depends. On Linux a few 10s of thousands

7. The big thing that goes wrong • Uncontrolled access to memory • race condition • with multiple writers of a single shared variable, updates can get lost • Fixed by: • single writers • locks

8. Before-and-after atomicity • Sometimes you need an arbitrary sequence of operations to be atomic • A sequence of operations that needs to be atomic is also called a critical section • Mutual exclusion means only one thread at a time (one thread in the critical section excludes all others)

9. Achieving mutual exclusion • In pthreads, mutual exclusion is provided by mutex objects • created as all other objects (malloc) • initialized by pthread_mutex_init() • acquired by pthread_mutex_lock() • released by pthread_mutex_unlock()

10. The lock idiom • Every critical section is protected by a mutex • The code looks like: pthread_mutex_lock(&lock); // critical section pthread_mutex_unlock(&lock);

11. Locking PingPong void *p(void *arg) { long i; for (i = 0; i < LIMIT; ++i) { pthread_mutex_lock(&lock); // Once the lock is held, this // “critical section” can be as long // as you need or want it to be counter = counter + 1; pthread_mutex_unlock(&lock); } return 0; }

12. Multiple critical sections • If there are multiple critical sections that access the same shared data • They need to be protected by the same lock

13. Multiple critical section idiom pthread_mutex_lock (&lock); // critical section // for thread 1 pthread_mutex_unlock (&lock); pthread_mutex_lock (&lock); // critical section // for thread 2 pthread_mutex_unlock (&lock);

14. Locking issues • Fine-grained locks • more parallelism • more overhead • more complexity • Coarse-grained locks • less parallelism • less overhead • simpler • Deadlock

15. Sample thread code • Lots of examples in the threads directory of the lectures repository

16. Using threads for speedup perfect speedup 45o line Speedup Cores used

17. Things to think about • Each core has its own cache • The L3 cache is shared between all the cores • If you have 8 cores, how many “pieces of work” should you create? • 8? • <8? • >8?

18. More things to think about • What if the “pieces of work” aren’t all the same size? • Or what if one thread is slower than the other threads? • Why could this be? • Randomness • Interference with other activity on the machine • These slow threads are called “stragglers” and are a real problem in practice

19. The ideal case – all threads finish at the same time

20. What might happen

21. Even more things to think about • Suppose I have 8 cores. • Should I create 8 threads – one for each core • Or more than 8 threads to deal with “stragglers” • Or ...