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Chapter 5: Threads

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  1. Chapter 5: Threads • Overview • Multithreading Models • Threading Issues • Pthreads • Solaris 2 Threads • Windows 2000 Threads • Linux Threads • Java Threads Operating System Concepts

  2. Thread vs. Process • A thread – lightweight process (LWP) is a basic unit of CPU utilization. • It comprises a thread ID, a program counter, a register set, and a stack. • A traditional (heavyweight) process has a single thread of control. • If the process has multiple threads of control, it can do more than one task at a time. Operating System Concepts

  3. Single and Multithreaded Processes Operating System Concepts

  4. Motivation • An application typically is implemented as a separate process with several threads of control. • For example, a web browser might have one thread display images or text while another thread retrieves data from the network. • It is more efficient for a process that contains multiple threads to serve the same purpose. • This approach would multithread the web-server process. Operating System Concepts

  5. Benefits • Responsiveness: Multithreading an interactive application may allow a program to continue running even if part of it is blocked or is performing a lengthy operation, thereby increasing responsiveness to the user. • Resource Sharing: Threads share the memory and the resources of the process to which they belong. • Economy: Allocating memory and resources for process creation is costly. • Utilization of MP (multiprocessor) Architectures: Each thread may be running in parallel on a different processor. Operating System Concepts

  6. User Threads • Thread management done by user-level threads library • User-level threads are fast to create and manage. • If the kernel is single-threaded, then any user-level thread performing a blocking system call will cause the entire process to block. • Examples - POSIX Pthreads - Mach C-threads - Solaris UI-threads Operating System Concepts

  7. Kernel Threads • Supported by the Kernel: The kernel performs thread creation, scheduling, and management in kernel space. • Examples - Windows 95/98/NT/2000 - Solaris - Tru64 UNIX - BeOS - OpenBSD - FreeBSD - Linux Operating System Concepts

  8. Multithreading Models • Many-to-One: The many-to-one model maps many user-level threads to one kernel thread. • One-to-One: The one-to-one model maps each user thread to a kernel thread. • Many-to-Many: The many-to-many multiplexes many user-level threads to a smaller or equal number of kernel threads. Operating System Concepts

  9. Many-to-One • Many user-level threads mapped to single kernel thread. • Used on systems that do not support kernel threads. Operating System Concepts

  10. Many-to-One Model Operating System Concepts

  11. One-to-One • Each user-level thread maps to kernel thread. • Examples - Windows 95/98/NT/2000 - OS/2 Operating System Concepts

  12. One-to-one Model Operating System Concepts

  13. 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 2 • Windows NT/2000 with the ThreadFiber package Operating System Concepts

  14. Many-to-Many Model Operating System Concepts

  15. Multithreading Models - Conclusion • Whereas the many-to-one model allows the developer to create as many user threads as she wishes, true concurrency is not gained because the kernel can schedule only one thread at a time. • The one-to-one model allows for greater concurrency, but the developer has to be careful not to create too many threads within an application. • The many-to-many model suffers from neither of these shortcomings: Developer can create as many user threads as necessary, and the corresponding kernel threads can run in parallel on a multiprocessor. Also, when a thread performs a blocking system call, the kernel can schedule another thread for execution. Operating System Concepts

  16. Threading Issues • Semantics of fork() and exec() system calls: Two versions of fork are one that duplicates all threads and another that duplicates only the thread that invoked the fork system call. • Thread cancellation is the task of terminating a thread before it has completed. • Two different scenarios are asynchronous cancellation and deferred cancellation. • A thread checks if it should be cancelled at a point referred as cancellation point. • Signal handling • Thread pools • Thread specific data is a copy of data owned by a thread. Operating System Concepts

  17. Signal Handling • A signal is used in UNIX systems to notify a process that a particular event has occurred. • All signals follow the same pattern: • A signal is generated by the occurrence of a particular event. • A generated signal is delivered to a process. • Once delivered, the signal must be handled. • A signal can be synchronous or asynchronous: • Synchronous signals are delivered to the same process. • Asynchronous signal is generated by an external event. Operating System Concepts

  18. Signal Handling • Every signal may be handled by one of two possible handlers: • A default signal handler • A user-defined signal handler • The options exist for delivering signals to a multithreaded program: • 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. • Windows 2000 uses asynchronous procedure calls (APCs) to emulate signals. Operating System Concepts

  19. Thread Pools • A multithread server has potential problems: • The time required includes thread creating and discarding. • Unlimited threads could exhaust system resources. • One solution to this issue is to use thread pools. • A thread pool has a number of threads being created at process startup, where they sit and wait for work. • The benefits of thread pools are: • It is usually faster to service a request with an existing thread. • A thread pool limits the number of threads at any one point. Operating System Concepts

  20. Thread-Specific Data • A multithread server has potential problems: • The time required includes thread creating and discarding. • Unlimited threads could exhaust system resources. • One solution to this issue is to use thread pools. • A thread pool has a number of threads being created at process startup, where they sit and wait for work. • The benefits of thread pools are: • It is usually faster to service a request with an existing thread. • A thread pool limits the number of threads at any one point. Operating System Concepts

  21. Pthreads • a POSIX standard (IEEE 1003.1c) 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. • Refer to the program shown in Figure 5.5. Operating System Concepts

  22. Solaris 2 Threads • Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, SMP, and real-time scheduling. • Solaris 2 implements the Pthread API and UI threads. • Between user- and kernel-level threads are ligthweight processes (LWPs). • Threads in a process multiplex to connect an LWP. An LWP corresponds a kernel thread. • A (un)bound user-level thread is (not) permanently attached to an LWP. Operating System Concepts

  23. Solaris 2 Threads Operating System Concepts

  24. Solaris Process Operating System Concepts

  25. Windows 2000 Threads • Implements the one-to-one mapping. • Each thread contains - a thread id - register set - separate user and kernel stacks - private data storage area Operating System Concepts

  26. 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) Operating System Concepts

  27. Java Threads • Java threads may be created by: • Extending Thread class • Implementing the Runnable interface • Calling the start method for the new object does two things: • It allocates memory and initializes a new thread in the JVM. • It calls the run method, making the thread eligible to be run by JVM. • Java threads are managed by the JVM. Operating System Concepts

  28. Java Thread States Operating System Concepts