Glossary - Nutt

Turnaround Time
In Computing Turnaround time is the total time taken between the submission of a program for execution and the return of the complete output to the customer.
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FCFS
First-come, first-served (sometimes first-in, first-served; first-come, first choice; or simply FCFS) is a service policy whereby the requests of customers or clients are attended to in the order that they arrived, without other biases or preferences.
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Gantt chart
A Gantt chart is a type of bar chart that illustrates a project schedule. Gantt charts illustrate the start and finish dates of the terminal elements and summary elements of a project. Terminal elements and summary elements comprise the work breakdown structure of the project. Some Gantt charts also show the dependency (i.e, precedence network) relationships between activities.
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SJN - Shortest Job Next(Shortest Job First)

Shortest job next is a scheduling policy that selects the waiting process with the smallest execution time to execute next.

Shortest job next is advantageous because of its simplicity and because it maximizes process throughput (in terms of the number of processes run to completion in a given amount of time). It also minimizes the average amount of time each process has to wait until its execution is complete. However, it has the potential for process starvation for processes which will require a long time to complete if short processes are continually added. Highest response ratio next is similar but provides a solution to this problem.

Shortest job next scheduling is rarely used outside of specialized environments because it requires accurate estimations of the runtime of all processes that are waiting to execute.

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Scheduling
Scheduling refers to the way processes are assigned to run on the available CPUs, since there are typically many more processes running than there are available CPUs. This assignment is carried out by softwares known as a scheduler and dispatcher.

The scheduler is concerned mainly with:

• CPU utilization - to keep the CPU as busy as possible.
• Throughput - number of processes that complete their execution per time unit.
• Turnaround - total time between submission of a process and its completion.
• Waiting time - amount of time a process has been waiting in the ready queue.
• Response time - amount of time it takes from when a request was submitted until the first response is produced.
• Fairness - Equal CPU time to each thread.

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RR - Round Robin

The scheduler assigns a fixed time unit per process, and cycles through them.

• RR scheduling involves extensive overhead, especially with a small time unit.
• Balanced throughput between FCFS and SJN, shorter jobs are completed faster than in FCFS and longer processes are completed faster than in SJN.
• Fastest average response time, waiting time is dependent on number of processes, and not average process length.
• Because of high waiting times, deadlines are rarely met in a pure RR system.
• Starvation can never occur, since no priority is given. Order of time unit allocation is based upon process arrival time, similar to FCFS.