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Threads

Threads. Chapter 4. Modern Process & Thread. Process is an infrastructure in which execution takes place  ( address space + resources) Thread is a program in execution within a process context – each thread has its own execution stack. Stack. Data. Thread. Thread. Stack. Process.

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Threads

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  1. Threads Chapter 4

  2. Modern Process & Thread • Process is an infrastructure in which execution takes place  (address space + resources) • Thread is a program in execution within a process context – each thread has its own execution stack Stack Data Thread Thread Stack Process … Program Stack Thread Operating System

  3. Single-threaded approaches Multithreaded Approaches MS-DOS Some variants of UNIX Windows, Solaris, modern versions of UNIX

  4. Thread (Lightweight Process) • An execution state (running, ready, etc.) • Saved thread context when not running • Has an execution stack • Some per-thread static storage for local variables • Access to the memory and resources of its process • all threads of a process share this

  5. Benefits of Threads • Takes less time to create a new thread than a process • Less time to terminate a thread than a process • Less time to switch between two threads within the same process • Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel Shared Address Space • All threads in a process implicitly use that process’s address space • Threads can share a program and data • If you want sharing, encode your work as threads in a process • Threads in separate processes can not facilitate sharing

  6. Thread Examples • Foreground to background work (spreadsheet: one thread reading user input, another executing the commands and updating the spreadsheet; yet another making periodic backups) • Asynchronous processing (ex: a thread performing periodic backups against power failures in a word-processor) • Fast execution (on a Multiprocessor system, multiple threads can execute in parallel) • Modular program structure (different tasks/activities in a program may be implemented using different threads) • Client/Server computing

  7. User-Level Threads (ULT) • All thread management is done by the application • The kernel is not aware of the existence of threads and schedules the process as a unit and assigns a single execution state (Ready, Running, Blocked, etc.) • Any application can be programmed to be multithreaded by using a threads librarywhich contains code for creating and destroying threads, for passing messages and data between threads, for scheduling thread execution, and for saving and restoring thread contexts. • Disadvantages: • when a ULT executes a blocking system call, all of the threads within the same process are blocked • can not take advantage of multiprocessing

  8. Kernel-Level Threads (KLT) • W2K, Linux, and OS/2 are examples of this approach • All of the work of thread management is done by the kernel. i.e. no thread management code in the program. • Kernel maintains context information for the process as a whole and for individual threads within the process • Scheduling is done by the kernel on a thread basis Advantages: • Kernel can simultaneously schedule multiple threads from the same process on multiple processors • If one thread in a process is blocked, the kernel can schedule another thread of the same process Disadvantage: transfer of control from one thread to the next in the same process requires a mode switch to the kernel

  9. Combined Approaches • Example: SUNSolaris • Thread creation, destruction, bulk of scheduling and synchronization are done in the user space. • multiple threads within the same application can run in parallel on multiple processors • if one is blocked, another thread within the same application need not block

  10. POSIX Thread Library (Linux) http://web.mst.edu/~ercal/284/ThreadExamples/thread_handout.doc • Thread creation pthread_create() • Synchronization pthread_mutex_lock(), pthread_mutex_unlock(), pthread_mutex_trylock() • Suspension pthread_cond_wait(), pthread_cond_signal(), pthread_join() • Yielding pthread_yield() • Termination pthread_exit()

  11. Symmetric Multiprocessing • Kernel can execute on any processor • Typically each processor does self-scheduling from the pool of available processes or threads

  12. Multiprocessor Operating System Design Considerations • Concurrent processes or threads • Synchronization • Scheduling • Memory Management • Reliability and Fault Tolerance

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