Module One - Session 04: Processes and Scheduling

What is a Process?

A process is the most fundamental parts of an OS. Without a process our OS would not be able to do anything. Here are a couple definitions of what a process is:

• A program in execution
• An instance of a program running on a computer
• The entity that can be assigned to and executed on a processor
• A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system resources

The OS needs to keep track of a lot of information/data related to each process. This information is stored in the Process Control Block .

Process States

There are also different states of a process. These are important to learn so we can gain a better understanding of how an Operating System works and is able to keep track of processes. Please read the Wikipedia entry on Process States .

Scheduling

Now that we understand how a process is created and the different states a process can be in we can now learn how a process is scheduled by the OS. Depending on the type of OS different scheduling algorithms are possible.

Batch Scheduling:

• First-Come-First-Served (FCFS) Scheduling
• Shortest-Job-First (SJF) Scheduling
• Shortest remaining time

Interactive Scheduling:

• Round Robin Scheduling
• Priority Scheduling
• Shortest-Job-First (SJF) Scheduling
• Guaranteed Scheduling: If N users logged in, you will receive 1/N CPU time. Single-user system with N processes each process get 1/N CPU time.
• Lottery scheduling
• Fair-share scheduling

Original webpage by Petersen Gross, modified by William Albritton.