Operating System

*Three primary thread libraries:

*POSIX Pthreads
*Win32 threads
*Java thread

Kernel Threads

Supported by the Kernel

Examples

Windows XP/2000

Solaris

Linux

Tru64 UNIX

Mac OS

The threads library allows concurrent programming in Objective Caml. It provides multiple threads of control (also called lightweight processes) that execute concurrently in the same memory space. Threads communicate by in-place modification of shared data structures, or by sending and receiving data on communication channels

-it is implemented by time-sharing on a single processor. It will not take advantage of multi-processor machines. Using this library will therefore never make programs run faster. However, many programs are easier to write when structured as several communicating processes.

The supports for threads can be provided either at the user level, called user threads, or by the kernel, called kernel threads, in which kernel threads are supported and managed directly by the OS. All contemporary OS support kernel threads. „ Ultimately, there must be a relationship between user threads and kernel threads, and there are three common ways of establishing relationships


Many user-level threads mapped to single kernel thread

Examples:

Solaris Green Threads

GNU Portable Threads

each user thread is mapped into one kernel thread. It provides maximum concurrency by allowing another thread to run when a thread (currently running) is blocked; It also allows multiple threads (within a process) to run in parallel on multiprocessors. The drawback is that the creation of a user thread requires the creation of the corresponding kernel thread. There is usually more overhead creating kernel thread than creating user thread, and most implementation of this model restricts the number of threads that can be supported by the system.

Each user-level thread maps to kernel thread

Examples

Windows NT/XP/2000

Linux

Solaris 9 and later

this typically allows many user-level threads to be mapped to a smaller or equal number of kernel threads, which is a hybrid model of the first two models. It provides better concurrency than many-to-one model (less than one-to-one model), yet is flexible in that it can create many user-threads not restricted by the number of kernel threads

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