Chapter 4: Threads Adapted to COP4610 by Robert van Engelen

4.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Process Versus Thread A process has its own address space, file descriptors of open files and devices, and other resources fork() duplicates the process A single process can have a single thread of control or multiple threads A new thread can be started at any time Each thread shares the same data, file descriptors, and code of the process A thread has its own registers, stack (for function calls), and program counter

4.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Single and Multithreaded Processes

4.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Benefits Benefits of multi-threading Responsiveness (e.g. main thread executes while another waits for I/O) Resource sharing Economy (threads are cheap compared to processes) Utilization of MP architectures For example, one thread of a Web browser renders the content of a page while another downloads data

4.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 User Threads Thread management done by user-level threads library Three primary thread libraries: POSIX Pthreads Win32 threads Java threads

4.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Kernel Threads Supported by the Kernel Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

4.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Multithreading Models Many-to-One One-to-One Many-to-Many

4.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

4.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

4.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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

4.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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

4.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

4.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads? Some systems provide two versions of fork One that copies all threads One that creates a process with a single thread

4.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation one thread terminates the target thread immediately Deferred cancellation allows the target thread to periodically check a flag if it should be cancelled  Allows a thread to cancel at a safe point, called a cancellation point in Pthreads

4.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred (e.g. control-C) 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 thread to receive all signals for the process

4.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Thread Pools Create a number of threads in a pool where they await work With more work than threads, work is queued until a thread fetches it from the queue 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

4.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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)

4.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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

4.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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)

4.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 C Pthread Example #include int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* the thread code, see next slide */ int main(int argc, char *argv[]) { pthread_t tid; /* the thread identifier */ pthread_attr_t attr; /* set of attributes for the thread */ int stat; /* the thread exit value */ if (argc != 2) { fprintf(stderr,"usage: a.out \n"); return -1; /* causes exit(-1); */ } if (atoi(argv[1]) < 0) { fprintf(stderr,"Argument %d must be non-negative\n",atoi(argv[1])); return -1; /* causes exit(-1); */ } pthread_attr_init(&attr); /* get the default attributes */ pthread_create(&tid,&attr,runner,argv[1]); /* create the thread */ pthread_join(tid,&stat); /* now wait for the thread to exit */ printf("sum = %d\n",sum); }

4.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 C Pthread Example (cont’d) /* 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); /* exit the thread with status 0 */ }

4.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Windows XP Threads Implements the one-to-one mapping Each thread contains A thread id Register set Separate user and kernel stacks Private data storage area The register set, stacks, and private storage area are known as the context of the threads The primary data structures of a thread include: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block)

4.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 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) Linux also supports Pthreads

4.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Java Threads Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface

4.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th edition, Jan 23, 2005 Java Thread States

End of Chapter 4