Process & Thread Management

Process

A process is a program under execution. The value of the program counter (PC) indicates the address of the next instruction of the process being executed. Each process is represented by a Process Control Block (PCB).

Process States

The state of a process is defined in part by the current activity of that process. A process can be in one of the following states:

New: The process is being created.
Ready: The process is waiting to be assigned to a processor.
Run: Instructions are being executed.
Block or wait: The process is waiting for some event to occur (such as an I/O completion or reception of a signal).
Completion or termination: The process has finished execution.
Suspend ready: When the ready queue becomes full, some processes are moved to suspend ready state.
Suspend wait or suspend block: When waiting queue becomes full.

Process Control Block (PCB)

Each process is represented in the operating system by a process control block (PCB) - also called a task control block. It contains many pieces of information associated with a specific process, including:

Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information

Thread

A thread is a lightweight process and forms the basic unit of CPU utilization. A process can perform more than one task at the same time by including multiple threads.

A thread has its own program counter, register set, and stack.
A thread shares resources with other threads of the same process: the code section, the data section, files and signals.
fork() system call: A new thread, or a child process of a given process, can be introduced by using the fork() system call. A process with n fork() system call generates 2^n – 1 child processes.
User threads: Implemented by users.
Kernel threads: Implemented by OS.

Difference between Process and Thread

Key differences between a process and a thread:

Process is heavy weight or resource intensive. Thread is light weight, taking fewer resources.
Process switching needs interaction with operating system. Thread switching does not need to interact with operating system.
In multiple processing environments, each process executes the same code but has its own memory and file resources. All threads can share same set of open files, child processes.
If one process is blocked, then no other process can execute until the first process is unblocked. While one thread is blocked and waiting, a second thread in the same task can run.
Multiple processes without using threads use more resources. Multiple threaded processes use fewer resources.
In multiple processes each process operates independently of the others. One thread can read, write or change another thread’s data.

Process Scheduling Terms

Key metrics used in process scheduling:

Arrival Time – Time at which the process arrives in the ready queue.
Completion Time – Time at which process completes its execution.
Burst Time – Time required by a process for CPU execution.
Turn Around Time – Time Difference between completion time and arrival time (TAT = CT - AT).
Waiting Time (WT) – Time Difference between turn around time and burst time (WT = TAT - BT).