Threads and Concurrency

• A thread is a flow of execution through the process code.

  • It has its own program counter that keeps track of which instruction to execute next.

  • It also has system registers which hold its current working variables,

  • and a stack which contains the execution history.

• A thread shares with its peer threads various information like code segment, data segment, and open files.

  • When one thread alters a code segment memory item, all other threads see that.

• A thread is also called a lightweight process.

  • Threads provide a way to improve application performance through parallelism.

  • Threads represent a software approach to improving the performance of operating systems by reducing the overhead.

  • A thread is equivalent to a classical process.

• Each thread belongs to exactly one process, and no thread can exist outside a process.

  • Each thread represents a separate flow of control.

  • Threads have been successfully used in implementing network servers and web servers.

  • They also provide a suitable foundation for parallel execution of applications on shared memory multiprocessors.

• Advantages of threads:

  • They minimize the context switching time.

  • Using them provides concurrency within a process.

  • They provide efficient communication.

  • It is more economical to create and context switch threads.

  • Threads allow utilization of multiprocessor architectures to a greater scale and efficiency.

• Threads are implemented in the following two ways:

  • User Level Threads: User-managed threads.

    • the thread management kernel is not aware of the existence of threads.

    • The thread library 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.

    • The application starts with a single thread.

  • Advantages:

    • Thread switching does not require Kernel mode privileges.

    • User level thread can run on any operating system.

    • Scheduling can be application-specific in the user level thread.

    • User level threads are fast to create and manage.

  • Disadvantages:

    • In a typical operating system, most system calls are blocking.

    • Multithreaded application cannot take advantage of multiprocessing.

  • Kernel Level Threads: Operating System-managed threads acting on a kernel, an operating system core.

    • thread management is done by the Kernel.

    • There is no thread management code in the application area.

    • Kernel threads are supported directly by the operating system.

    • Any application can be programmed to be multithreaded.

    • All of the threads within an application are supported within a single process.

    • The Kernel maintains context information for the process as a whole and for individuals threads within the process.

    • Scheduling by the Kernel is done on a thread basis.

    • The Kernel performs thread creation, scheduling, and management in Kernel space.

    • Kernel threads are generally slower to create and manage than the user threads.

    • Advantages

      • The Kernel can simultaneously schedule multiple threads from the same process on multiple processes.

      • If one thread in a process is blocked, the Kernel can schedule another thread of the same process.

      • Kernel routines themselves can be multithreaded.

    • Disadvantages

      • Kernel threads are generally slower to create and manage than the user threads.

      • Transfer of control from one thread to another within the same process requires a mode switch to the Kernel.