Lightweight processes and threads concurrency level

The SVR4.2MP and UnixWare 2.0 operating system kernel is not aware of the multithreading of any process using the Threads Library. The kernel supports an entity known as a ``lightweight process'' (LWP).

• There may be many LWPs associated with a single process.

• Each LWP of a process shares the process address space with its ``sibling'' LWPs.

• Each LWP has its own scheduling context.

• On multiprocessor machines, several LWPs of a process might be running on different processors simultaneously (true concurrency).

• Each LWP has access to all of the resources of the process such as open file descriptors, access rights and privileges, and resource limits.

Many of these are features of threads as well. This is no coincidence. Threads have many of these features because a thread only executes once it has been ``picked up'' by an LWP. However, a process typically has more threads than LWPs.

Conceptually, an LWP is a dearer resource than a thread. The Threads Library will typically maintain a pool of LWPs that are shared by the set of runnable threads in a process.