Thread Models in Operating System

A thread is a light weight process which is similar to a process where every process can have one or more threads. Each thread contains a Stack and a Thread Control Block. There are four basic thread models : 

1. User Level Single Thread Model :

• Each process contains a single thread.
• Single process is itself a single thread.
• process table contains an entry for every process by maintaining its PCB.

2. User Level Multi Thread Model :

• Each process contains multiple threads.
• All threads of the process are scheduled by a thread library at user level.
• Thread switching can be done faster than process switching.
• Thread switching is independent of operating system which can be done within a process.
• Blocking one thread makes blocking of entire process.
• Thread table maintains Thread Control Block of each thread of a process.
• Thread scheduling happens within a process and not known to Kernel.

3. Kernel Level Single Thread Model :

• Each process contains a single thread.
• Thread used here is kernel level thread.
• Process table works as thread table.

4. Kernel Level Multi Thread Model :

• Thread scheduling is done at kernel level.
• Fine grain scheduling is done on a thread basis.
• If a thread blocks, another thread can be scheduled without blocking the whole process.
• Thread scheduling at Kernel process is slower compared to user level thread scheduling.
• Thread switching involves switch.

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