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1.Process Scheduling Cheat Sheet by

Process Scheduling is done in Operating System to allocate CPU to the processes.

Sche­duling Queue

Job Queue: A process when enters a system is put into a job queue.
Ready Queue: The process residing in the main memory and ready for execution is put into ready queue.
Device Queue: The process waiting for a particular I/O device is put in the device queue.

Sche­duler

Long Term Schedu­ler: Also known as job scheduler selects process from disk and puts into the main memory.
Short Term Schedu­ler: Also known as CPU Scheduler selects process from memory and allocates a CPU to it.
Medi­um-Term Schedu­ler: It is used to remove a process and reduce the degree of multi-­pro­graming . Later it can be re-int­roduced from point where it was left .This is known as Swapp­ing.

Priority Scheduling Round Robin Schedu­ling

Each process has a prior­ity.
There is a fixed time quantum.
CPU allocated to the process with higher priority.
Ready queue is treated as circular queue and CPU is allocated to the First process for specific time quantum.
Prob­lem: Starv­ation of low priority. Solu­tion: Aging.
Prob­lem: If time quantum too large, algorithm works as FCFS.
It can be preem­ptive or non-p­ree­mpt­ive.
It is preem­ptive.

Mult­ilevel Queue Multilevel Feedback Queue

Ready queue is divided into: Foreg­round (inter­active) process and Backg­round (batch) process.
Allows the process from one queue to move to the next queue.
Fore­gro­und implements Round Robin Schedu­ling and Back­gro­und implements FCFS.
Here processes are separated according to their CPU burst.
 

Sche­duling Criteria

1. CPU Utiliz­ati­on: It should be maximum. 40% minimum- 90% maximum.
2. Throug­hput: Number of processes completed per unit time is called throu­ghput. It should be minimum.
3. Turnaround Time: The interval from time of submission of process to time of comple­tion, Turna­round Time= period spent waiting + waiting in ready queue + execution time + I/O interrupt time. It should be minimum.
4. Waiting Time: The time for which the process has to wait in the ready queue is waiting time. It should be minimum.
5. Response Time: Time taken to respond to a process is a response time. It should be minimum.

SCHE­DULING ALGORI­THM

Scheduling Algorithm decides which process should the CPU be allocated to. There are six scheduling algori­thms.

First Come First Serve Shortest Job First

The process that requests for the CPU first, gets the access first.
Each process has the length of the next CPU burst.
FIFO queue is used in handling the process.
The process with smallest next CPU burst gets access to the process.
Long waiting time for the next process.
Compar­itively less waiting time for the next process.
It is a non-p­ree­mptive algorithm.
It can be preem­ptive or non-p­ree­mpt­ive.

Priority Scheduling Diagram

Round Robin Diagram

 

Sche­duling Queue Diagram

Medi­m-Term Scheduling Diagram

First Come First Serve (FCFS) Diagram

Shortest Job First (SJF) Diagram

Mult­ilevel Queue Scheduling Diagram

Mult­ilevel Feedback Queue Diagram

       

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