Process Part2 Printable.pdf

Processes - Part II Amir H. Payberah [email protected] Sep. 13, 2023 Threads 1 / 42 Thread A basic unit of CPU utilization. https://tinyurl.com/e8crhtne 2 / 42 Threads (1/2) I A traditional process: has a single thread. I Multiple threads in a process: performing more than one task at a ti...

Processes - Part II Amir H. Payberah [email protected] Sep. 13, 2023 Threads 1 / 42 Thread A basic unit of CPU utilization. https://tinyurl.com/e8crhtne 2 / 42 Threads (1/2) I A traditional process: has a single thread. I Multiple threads in a process: performing more than one task at a time. I Threads in a process share code section, data section, and other OS resources, e.g., open files. 3 / 42 Threads (2/2) I Multiple tasks of an application can be implemented by separate threads. • • • • Update display Fetch data Spell checking Answer a network request 4 / 42 Threads - Example I Multi-threaded web-server architecture 5 / 42 Threads Benefits I Responsiveness: allow continued execution if part of process is blocked. I Resource Sharing: threads share resources of process, easier than shared memory or message passing. I Economy: thread switching has lower overhead than context switching. I Scalability: process can take advantage of multiprocessor architectures. 6 / 42 Multi-core Programming 7 / 42 Multi-core Systems I Users need more computing performance: single-CPU → multi-CPU I A similar trend in system design: multi-core systems • I Each core appears as a separate processor. Multi-threaded programming • Improves concurrency and more efficient use of multiple cores. 8 / 42 Concurrency vs. Parallelism (1/2) I Concurrency: supporting more than one task by allowing all the tasks to make progress. • I A scheduler providing concurrency. Concurrent execution on a single-core system. 9 / 42 Concurrency vs. Parallelism (2/2) I Parallelism: performing more than one task simultaneously. I Parallelism on a multi-core system. 10 / 42 Types of Parallelism I Data parallelism • I Distributes subsets of the same data across multiple cores, same operation on each. Task parallelism • Distributes threads across cores, each thread performing unique operation. 11 / 42 Multi-threading Models 12 / 42 User Threads and Kernel Threads I User threads: managed by user-level threads library. • • • • I Three primary thread libraries: POSIX pthreads Windows threads Java threads Kernel threads: supported by the Kernel. 13 / 42 Multi-Threading Models I Many-to-One I One-to-One I Many-to-Many 14 / 42 Many-to-One Model I Many user-level threads mapped to single kernel thread. I One thread blocking causes all to block. I Multiple threads may not run in parallel on multi-core system because only one may be in kernel at a time. I Few systems currently use this model. • • Solaris green threads GNU portable threads 15 / 42 One-to-One Model I Each user-level thread maps to one kernel thread. I Creating a user-level thread creates a kernel thread. I More concurrency than many-to-one. I Number of threads per process sometimes restricted due to overhead. I Examples: • • Windows Linux 16 / 42 Many-to-Many Model I Allows many user-level threads to be mapped to many kernel threads. I Allows the OS to create a sufficient number of kernel threads. I Examples: • • Windows with the ThreadFiber package Otherwise not very common 17 / 42 Thread Libraries 18 / 42 Thread Libraries (1/2) I Thread library provides programmer with API for creating and managing threads. I Two primary ways of implementing: • • Library entirely in user-space. Kernel-level library supported by the OS. 19 / 42 Thread Libraries (2/2) I Pthread • I Windows thread • I Either a user-level or a kernel-level library. Kernel-level library. Java thread • Uses a thread library available on the host system. 20 / 42 21 / 42 Pthreads I A POSIX API for thread creation and synchronization. I Specification, not implementation. I API specifies behavior of the thread library, implementation is up to development of the library. I Common in UNIX OSs, e.g., Solaris, Linux, Mac OS X 22 / 42 Thread ID I The thread ID (TID) is the thread analogue to the process ID (PID). I The PID is assigned by the Linux kernel, and TID is assigned in the Pthread library. I Represented by pthread t. I Obtaining a TID at runtime: #include <pthread.h> pthread_t pthread_self(void); 23 / 42 Creating Threads I pthread create() defines and launches a new thread. #include <pthread.h> int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*thread_func)(void *), void *arg); I thread func has the following signature: void *thread_func(void *arg); 24 / 42 Terminating Threads I Terminating yourself by calling pthread exit(). #include <pthread.h> void pthread_exit(void *retval); I Terminating others by calling pthread cancel(). #include <pthread.h> int pthread_cancel(pthread_t thread); 25 / 42 Joining and Detaching Threads I Joining allows one thread to block while waiting for the termination of another. I You use join if you care about what value the thread returns when it is done, and use detach if you do not. #include <pthread.h> int pthread_join(pthread_t thread, void **retval); int pthread_detach(pthread_t thread); [https://computing.llnl.gov/tutorials/pthreads/#Joining] 26 / 42 A Threading Example void *thread_func(void *message) { printf("%s\n", (const char *)message); return message; } int main(void) { pthread_t thread1, thread2; const char *message1 = "Thread 1"; const char *message2 = "Thread 2"; // Create two threads, each with a different message. pthread_create(&thread1, NULL, thread_func, (void *)message1); pthread_create(&thread2, NULL, thread_func, (void *)message2); // Wait for the threads to exit. pthread_join(thread1, NULL); pthread_join(thread2, NULL); return 0; } 27 / 42 Implicit Threading 28 / 42 Implicit Threading I Increasing the number of threads: program correctness more difficult with explicit threads. I Implicit threading: creation and management of threads done by compilers and run-time libraries rather than programmers. I Four methods explored: • • • Thread Pools Fork-Join OpenMP 29 / 42 Thread Pools I Create a number of threads in a pool where they await work. I Usually slightly faster to service a request with an existing thread than create a new thread. I Allows the number of threads in the application(s) to be bound to the size of the pool. 30 / 42 Fork-Join (1/2) I Multiple threads (tasks) are forked, and then joined. 31 / 42 Fork-Join (2/2) 32 / 42 OpenMP (1/2) I Set of compiler directives and APIs for C, C++, FORTRAN. I Identifies parallel regions: blocks of code that can run in parallel. I #pragma omp parallel: create as many threads as there are cores. I #pragma omp parallel for: run for loop in parallel. 33 / 42 OpenMP (2/2) #include <omp.h> #include <stdio.h> int main(int argc, char *argv[]) { /* sequential code */ #pragma omp parallel { printf("I am a parallel region."); } /* sequential code */ return 0; } 34 / 42 Thread Cancellation 35 / 42 Thread Cancellation (1/4) I Terminating a thread before it has finished. I Thread to be canceled is target thread. I Two general approaches: • • Asynchronous cancellation terminates the target thread immediately. Deferred cancellation allows the target thread to periodically check if it should be cancelled. 36 / 42 Thread Cancellation (2/4) int counter = 0; pthread_t tmp_thread; int main() { pthread_t thread1, thread2; pthread_create(&thread1, NULL, thread_func1, NULL); pthread_create(&thread2, NULL, thread_func2, NULL); pthread_join(thread1, NULL); pthread_join(thread2, NULL); } 37 / 42 Thread Cancellation (3/4) void* thread_func2(void* args) { tmp_thread = pthread_self(); while (1) { printf("thread number two\n"); sleep(1); // sleep 1 second } } 38 / 42 Thread Cancellation (4/4) void* thread_func1(void* args) { while (1) { printf("thread number one\n"); sleep(1); counter++; if (counter == 2) { pthread_cancel(tmp_thread); pthread_exit(NULL); } } } 39 / 42 Summary 40 / 42 Summary I Single-thread vs. Multi-thread I Concurrency vs. parallelism I Multi-threading models: many-to-one, one-to-one, many-to-many I Multi-thread libraries: pthread I Implicit threading I Thread cancellation 41 / 42 Questions? Acknowledgements Some slides were derived from Avi Silberschatz slides. 42 / 42