4.3 Multithreading Models

User-level threads

Threads managed without kernel support.

Kernel-level threads

Threads managed directly by the operating system.

Many-to-One Model

Multiple user threads are mapped to a single kernel thread.

Drawback of Many-to-One Model

If any thread makes a blocking system call, the entire process blocks.

One-to-One Model

Each user thread is mapped to a separate kernel thread.

Advantage of One-to-One Model

Allows greater concurrency and can run multiple threads in parallel on multiprocessors.

Many-to-Many Model

Multiple user threads are mapped to a smaller or equal number of kernel threads.

Advantage of Many-to-Many Model

Allows for many user threads without blocking parallelism.

Two-Level Model

A variation of the many-to-many model where user threads are bound to kernel threads.

Current Trend in Multithreading

Most contemporary OS use the One-to-One Model for its balance of concurrency and simplicity.