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Scheduling Criteria, Terms, and Concepts

(i) CPU utilization:

We want to utilize the CPU as much as possible i.e. we are always trying ranges to reduce the CPU idealness.CPU utilization range from 0 to 100 percent. In practice, it should range from 40 percent (for a lightly loaded system) to 90 percent (for a heavily loaded system).

(ii) Throughput:

CPU is busy to execute and complete the processes (work). One measure of work is the number of processes that are completed per unit time, called throughput (performance of CPU). For long processes, this rate may be one process per hour; for short transactions, and it may be ten processes per second.

A multiprocessor OS exploits the presence of multiple CPUs in the computer to provide high throughput.

(iii) Arrival Time (AT):

The time when the process has arrived in the ready queue (main memory) is called the arrival time of a process.

(iv) Completion Time (CT):

The time when the process is completed its total execution is called the completion time of a process.

(v) Burst Time (BT) or Service Time (ST):

The time required by the process for its execution is called burst time or execution time of a process. We can also say the total CPU time and I/O time required by a job, the process to complete its operation.

(vi) Turn Around Time (TAT):

The interval from the time of submission of a process to the time of completion is called turnaround time. Turnaround time is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.

Turn Around Time = Completion Time (CT) - Arrival Time (AT)

Turn Around Time = Burst Time (BT) + Waiting Time (WT)

Average Turn Around Time = Sum of TAT of all processes / Number of processes

(vii) Waiting Time (WT):

The CPU-scheduling algorithm does not affect the amount of time during which a process executes or does I/O. It affects only the amount of time that a process spends waiting in the ready queue. Waiting time is the sum of the periods spent waiting in the ready queue.

Waiting Time (WT) = Turn Around Time (TAT) – Burst Time (BT)

Average Waiting Time = Sum of WT of all processes / Number of processes

(viii) Response Time (RT):

The time difference between the first response and arrival time is called the response time of a process.

Response Time (RT) = FR (First Response) – AR (Arrival Time)

Scheduling Terms and Concepts Category wise

Term or concept	Definition or description
Request related
Arrival time	Time when a user submits a job or process.
Admission time	Time when the system starts considering a job or process for scheduling.
Completion time	Time when a job or process is completed.
Deadline	Time by which a job or process must be completed to meet the response requirement of a real-time application.
Service time	The total CPU time and I/O time required by a job, process, or sub request to complete its operation.
Preemption	Forced deallocation of CPU from a job or process.
Priority	A tie-breaking rule used to select a job or process when many jobs or processes await service.
User service related: individual request
Deadline overrun	The amount of time by which the completion time of a job or process exceeds its deadline. Deadline overruns can be either positive or negative.
Fair share	A specified share of CPU time should be devoted to the execution of a process or a group of processes.
Response ratio	The ratio: (time since arrival + service time of a job or process) / Service time of the job or process
Response time (rt)	Time between the submissions of a job for processing to the time its result becomes available. This concept is applicable to interactive processes. The most important one is to minimize response time, that is, the time between issuing a command and getting the result. On a personal computer where a background process is running (for example, reading and storing email from the network), a user request to start a program or open a file should take precedence over the background work. Having all interactive requests go first will be perceived as good service.
Turnaround time (ta)	Time between the submission of a job or process and its completion by the system. This concept is meaningful for non-interactive jobs or processes only.
Weighted turnaround (w)	Ratio of the turnaround time of a job or process to its own service time.
User service related: average service
Mean response time (rt)	Average of the response times of all sub requests serviced by the system.
Mean turnaround time (ta)	Average of the turnaround times of all jobs or processes serviced by the system.
Performance related
Schedule length	The time taken to complete a specific set of jobs or processes.
Throughput	The average number of jobs, processes, or sub requests completed by a system in one unit of time. Turnaround time is the statistically average time from the moment that a batch job is submitted until the moment it is completed. It measures how long the average user has to wait for the output. Example: 6 process completed in 60 times then throughput = 6 / 60 = 0.1.
CPU utilization	This is the percentage of time that the CPU is busy. Processor Utilization = Processor Busy Time / Processor Busy Time + Processor Idle Time
GANTT Chart	GANTT (Generalized Activity Normalization Time Table) Chart: It is a horizontal bar chart that illustrates a project or processes schedule.
Overhead

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