Chapter 4: Threads

4.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threads A thread (or lightweight process) is a basic unit of CPU utilization; it consists of: program counter register set stack space A thread shares with its peer threads its: code section data section operating-system resources A traditional or heavyweight process is equal to a task with one thread

4.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threads (Cont.) In a multiple threaded process, while one server thread is blocked and waiting, a second thread in the same process can run. Cooperation of multiple threads in same job confers higher throughput and improved performance. Applications that require sharing a common buffer (i.e., producer- consumer) benefit from thread utilization. Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism. Kernel-supported threads (Mach and OS/2). User-level threads; supported above the kernel, via a set of library calls at the user level (Project Andrew from CMU). Hybrid approach implements both user-level and kernel-supported threads (Solaris 2).

4.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multiple Threads within a Process (task)

4.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Single and Multithreaded Processes

4.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Responsiveness Resource Sharing Economy Utilization of MP Architectures

4.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Thread management is done by user-level threads library Three primary thread libraries: POSIX Pthreads Win32 threads Java threads

4.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kernel Threads Supported by the Kernel Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

4.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multithreading Models Many-to-One One-to-One Many-to-Many

4.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

4.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Model

4.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

4.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-one Model

4.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

4.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model

4.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Two-level Model Similar to M:M, except that it allows a user thread to be bound to kernel thread Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier

4.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Two-level Model

4.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

4.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads?

4.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

4.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals  Signal is generated by particular event  Signal is delivered to a process  Signal is handled Options: Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific threa to receive all signals for the process

4.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Pools Create a number of threads in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool

4.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Specific Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

4.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number kernel threads

4.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthreads A POSIX standard (IEEE c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X)

4.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthread Example /** * A pthread program illustrating how to * create a simple thread and some of the pthread API * This program implements the summation function where * the summation operation is run as a separate thread. * * Compilation: * gcc –lpthread thrd-posix.c * */ #include int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* the thread */

4.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthread Example (cont.) int main(int argc, char *argv[]) { pthread_t tid; /* the thread identifier */ pthread_attr_t attr; /* set of attributes for the thread */ if (argc != 2) { fprintf(stderr,"usage: a.out \n"); return -1; } if (atoi(argv[1]) < 0) { fprintf(stderr,"Argument %d must be non-negative\n",atoi(argv[1])); return -1; } /* get the default attributes */ pthread_attr_init(&attr); /* create the thread */ pthread_create(&tid,&attr,runner,argv[1]); /* now wait for the thread to exit */ pthread_join(tid,NULL); printf("sum = %d\n",sum); return 0; }

4.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthread Example (cont.) /** * The thread will begin control in this function */ void *runner(void *param) { int i, upper = atoi(param); sum = 0; if (upper > 0) { for (i = 1; i <= upper; i++) sum += i; } pthread_exit(0); }

4.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Linux Threads Linux refers to them as tasks rather than threads Thread creation is done through clone() system call clone() allows a child task to share the address space of the parent task (process)

4.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Threads Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface studied in the course: Advanced Programming

4.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread States

End of Chapter 4