Multithreading Tutorial Professor: Shu-Ching Chen TA: Hsin-Yu Ha

What is a Thread? An independent stream of instructions that can be scheduled to run A path of execution int a, b; int c; a = 1; b = a + 2; c = 3; Sequentially execute c = 5; CPU Multi-Thread Single-Thread

Program v.s. Process v.s. Thread An execution file stored in the harddrive Process An execution file stored in the Memory Thread An execution path of part of the process HDD Memory Program Process Thread

Why is thread? Parallel execution Shared resources Easier to create and destroy than processes (100X) Easy porting to multiple CPUs

Compiling a pthreads program Return Result :

Creating threads (1) pthread_create() Return 0 if OK, nonzero on error Four Arguments Thread : A thread identifier Attr : A pointer to a thread attribute object Start_routine : A pointer to the function the thread is execute Arg : The argument to the function

Creating threads (2) Single variable

Creating threads (3) Several variables

Terminating Threads (1) pthread_exit() Return 0 if OK, nonzero on error Four ways of terminating a thread The thread returns from its starting routine The thread makes a call to the pthread_exit subroutine The thread is canceled by another thread The entire process is terminated

Terminating Threads (2) Example of pthread_exit()

Terminating Threads (3) pthread_join() Return 0 if OK, nonzero on error Wait from other threads to terminate by calling it

Terminating Threads (4)

Other Thread functions pthread_self() It returns the unique, system assigned thread ID of the calling thread pthread_detach() Return 0 if OK, nonzero on error It can be used to explicitly detach a thread

Synchronizations Join Semaphore Mutexes Barrier

Synchronization - Semaphore (1) A separate POSIX standard does define them Important functions sem_init : Initialize a new semaphore sem_destroy : deallocate an existing semaphore sem_wait : sem_post

Synchronization - Semaphore (2)

Synchronization - Mutexes (1) Mutexes are used to prevent data inconsistencies due to operations by multiple threads upon the same memory area performed at the same time to prevent race conditions where an order of operation upon the memory is expected

Synchronization - Mutexes (2)

Synchronization - Mutexes (3)

Synchronization – Barrier (1) It is used when some parallel computations need to "meet up" at certain points before continuing. Pthreads extension includes barriers as synchronization objects (available in Single UNIX Specification) Enable by #define _XOPEN_SOURCE 600 at start of file Initialize a barrier for count threads int pthread_barrier_init(pthread_barrier_t *barrier, const pthread_barrier attr_t *attr, int count); Each thread waits on a barrier by calling int pthread_barrier_wait(pthread_barrier_t *barrier); Destroy a barrier int pthread_barrier_destroy(pthread_barrier_t *barrier);

Synchronization – Barrier (2)

References POSIX Threads Programming Pthreads primer https://computing.llnl.gov/tutorials/pthreads/ Pthreads primer http://pages.cs.wisc.edu/~travitch/pthreads_primer.html POSIX thread (pthread) Tutorial http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html