Transcript Document
SMP threads an Introduction to Posix Threads Technical Definition 1. Independent stream of instructions that can be scheduled to run by an operating system 2. The smallest unit of processing that can be run by an operating system 3. Typically a thread is contained within a process Shared Memory API's 1. OpenMP o An open source solution to the threaded problem overseen by a board of industry heads o an explicit set of compiler directives which require compiler support o A very programmer friendly API o Less control over execution than other APIs 2. Posix threads o A international standard o A more complicated API base o More control over execution than other APIs 3. Kthreads o Kernel level implementation of threads Shared Memory Model Threads have their own private data and access to shared memory of other threads in their process. Structures of SMP Programs How does Linux Handle Threading? The kernel handles the creation of threads very similar to the way it handles the creation of processes (the clone() function), However the use of specific flags will describe what specific resources are to be shared. This means that processes are also threads and threads are simply a special brand of process tied to another thread. All processes and threads are of the type task_struct Pthread API • 100 different subroutines • 9 different prefixes of subroutines o pthread_ o pthread_attr_ o pthread_mutex_ o pthread_mutexattr_ o pthread_cond_ o pthread_condattr_ o pthread_key_ o pthread_rwlock_ o pthread_barrier_ pthread_create( pthread_t *, const pthread_attr_t *, void*(*)(void *), void *); pthread_t * is a pointer to a pthread_t object you allocate to your stack pthread_attr_t is what kind of flags will be set that define how this thread will behave with other threads in the process (passing null is allowed) void*(*)(void *) is the name of a function which will return a void * and has only 1 argument, a void pointer void * is a pointer to a structure of your arguments or a cast of a single argument pthread_attr_ pthread_attr_t is the data type that will be passed to pthread_create to describe how the pthread should interact with other threads Generally the pthread_attr_t is allocated on the stack and passing the address to your functions is acceptable There are several different flags that can be passed into the this attribute with several different functions. The most useful of these being pthread_attr_init() pthread_attr_setdetachstate() and PTHREAD_CREATE_JOINABLE Critical Sections Critical Sections of code are defined as: o Code paths that manipulate shared data This means we must protect our programs from having multiple threads in their critical sections Protecting Critical Sections o Mutexes (Mutual Exclusion) works by accessing a shared mutex which blocks when its already in use and acquires when its not in use o Semaphores work by letting decrementing a counter (atomically) if that would put the semaphore to a negative value it blocks and sleeps the process. Questions?