1. http://aqualab.cs.northwestern.edu Multithreaded Web Server

2. Rula et al. @ AquaLab 2 Threads –Allow multiple computation streams to run “in parallel” –Great for operations with blocking calls to the network or IO Types of threads –Kernel Threads –User Threads –Mapping between them – Linux 1:1

3. Rula et al. @ AquaLab 3 Synchronization –Allow resource and access control to multiple concurrent processes Semaphore –Send signals between processes –Binary, counting Mutex –Mutual Exclusion –Traditional Lock Condition Variables

4. Rula et al. @ AquaLab 4 struct sem { mutex m; condvar cv; unsigned int count; }; sem_init(s, value) { mutex_init(s.m); condvar_init(s.cv); count = value; } wait(s) { mutex_lock(s.m); while (s.count <= 0) { condvar_wait(s.cv, s.m); } --s.count; mutex_unlock(s.m); } post(s) { mutex_lock(s.m); ++s.count; condvar_broadcast(s.cv) mutex_unlock(s.m); }

5. Rula et al. @ AquaLab 5 Threadpools allow finite threads to be reused across multiple streams of execution Better performance –Eliminates overhead of thread creation and destruction –Reuse existing threads Typically consist of Worker Queue and Pool of Created Threads

6. Rula et al. @ AquaLab 6 Task Queue Thread Pool Completed Tasks

7. Rula et al. @ AquaLab 7 Task Queue Thread Pool Completed Tasks

8. Rula et al. @ AquaLab 8 POSIX Pthreads library core functions: Go here for tutorial: https://computing.llnl.gov/tutorials/pthreads/https://computing.llnl.gov/tutorials/pthreads/ Threads pthread_create(thread,attr,start_routine,arg) pthread_exit(status) pthread_cancel(thread) pthread_join(threadid, status) Mutex pthread_mutex_init(mutex,attr) pthread_mutex_destroy(mutex) pthread_mutex_lock(mutex) pthread_mutex_unlock(mutex) Condition Variable pthread_cond_init(condition, attr) pthread_cond_destroy(condition) pthread_cond_wait(condition, mutex) pthread_cond_signal(condition)

9. Rula et al. @ AquaLab 9 Create thread pool –Easiest structure Worker queue with pending tasks Each threads running in loop When job is added to the pool, pthread_cond_signal(&(pool->notify)) Add locking to seats.c Standby List –Implement semaphore –More on this Check for job in queue  if job, pull and run  else wait on condition variable  broadcast notify  Check for job in queue...

10. Rula et al. @ AquaLab 10 Adding multithreading to webserver While(1) { connfd = accept(listenfd, (struct sockaddr*)NULL, NULL); //without threads handle_connection(connfd); //with threads pool_add(threadpool, (void *) &handle_connection, (void *) connfd, 0); }

11. Rula et al. @ AquaLab 11 Condition Variables –Should be in a while loop to prevent race conditions Check for spurious wake-ups. When returning from pthread_cond_wait(), you own lock while((pool->count == 0) && (!pool->shutdown)) { pthread_cond_wait(&(pool->notify), &(pool->lock)); }

12. Rula et al. @ AquaLab 12 Only use a binary semaphore on the standby list –Implement semaphore but use it as a lock... –On the standby list data structure only Order of operations w/ standby list –When thread completes a job / wakes from condition variable, it first checks the standby list in the case that the previous operation freed the seat. If seat is Available, assign seat to that user If seat is Pending, do nothing If seat is Occupied, remove entry from the standby list