Process management

The state of managing active processes running in a computer system.

Terminologies

Term	Definition
Arrival time (AT)	The time a process arrives at the ready queue
Burst time (BT)	The estimated time for the CPU to execute a process
Finish time (FT)	The absolute time a process has finished executing
Turnaround time (TT)	The total time between a process’s arrival time and finish time ($TT=FT-AT$)
Waiting time (WT)	The total time a process waits to be executed ($WT=TT-BT$)
Throughput	The number of processses completed per unit time

Scheduling schemes

A process scheduler is a component that arranges and queues processes to be executed by a single CPU. Its main roles are to:

• refer to built-in policies and procedures to decide which processes to run first and which processes to be given the chance to finish first; and
• be responsible for scheduling the next process in the ready queue to be executed.

Allocation schemes

Different managers will schedule and execute processes in different scheduling schemes that they support. Popular schemes include:

• first-come first-serve (FCFS);
• round robin (RR);
• shortest job next (SJN); and
• shortest remaining time first (SRTF).

Scheme	Definition
First-come first-serve	A scheduling algorithm where tasks are enqueued and the process at the front of the queue is run.
Round robin	A scheduling algorithm similar to first-come first-serve, but rotates across the processes.

Allocation schemes have a direct impact on the efficiency of the CPU. No single allocation scheme is optimal; each has its own design goals. There are two kinds of allocation schemes:

• non-preemtive, where a process’s execution is not allowed to be interrupted; and
  • example: FCFS
• preemptive, where a process’s execution is allowed to be interrupted.
  • example: RR

Benefits and disadvantages

• FCFS
  • [+] Simple to use
  • [+] Fair (provided no long processes are present)
  • [-] Stalling and hogging of the CPU by long processes
• RR
  • [+] Fair
  • [+] Able to incorporate other schemes
  • [-] Practically equal to FCFS if time quantum is too small
  • [-] A lot of context switching present when the time quantum is too small

States

Processes can exist in four different states:

• ready state;
  • processes in this state are ready for execution by the CPU. In this instance, they are placed in a ready queue
  • some processes may be returned to the ready state after running or waiting before execution is completed
• waiting state;
  • processes in this state are put on hold, often waiting for some part of the OS to respond to a request by the process
  • not a mandatory state
• running state; and
  • processes in this state are being actively executed by the CPU
  • processes may move on or back to the finished, ready, or waiting state depending on the context
• finished state.
  • processes in this state have been executed. System resources it was allocated with are released, and the process terminates

Commands

Windows	Linux	Description
tasklist	ps	Lists the curerntly running processes in a computer
taskkill	kill	Stops (kills) a running process, usually the given process