In CPU scheduling, what are throughput, turnaround time, waiting time and response time, and how do they describe system performance?

Introduction / Context:
Scheduling algorithms in operating systems are evaluated using several standard performance metrics. These include throughput, turnaround time, waiting time and response time. Understanding what each of these terms means and how they relate to user experience and system utilization is essential for solving exam problems and for reasoning about scheduling design choices.

Given Data / Assumptions:

• We are considering jobs or processes that arrive, execute and complete on a CPU.

• The operating system uses a scheduling algorithm to decide which job runs when.

• Time is measured from the moment a job enters the system until various events occur.

• The question asks for correct definitions of four standard metrics.

Concept / Approach:
Throughput measures how many jobs can be completed in a given period, often jobs per second or per hour. Turnaround time measures how long a specific job takes from arrival to completion, including all waiting, running and I/O periods. Waiting time is the total amount of time a job spends in the ready queue or queues, not doing useful work on the CPU. Response time measures how long it takes from submission of a request until the system produces the first visible response, which is especially important for interactive systems. The correct option should give definitions that match these standard meanings.

Step-by-Step Solution:
Step 1: Recall that throughput is a system wide metric: number of completed jobs divided by the length of the observation interval. Step 2: Turnaround time for a job is completion time minus arrival time, covering the entire life cycle in the system. Step 3: Waiting time for a job is the sum of all time periods it spends in ready state waiting for CPU, not counting actual CPU execution or I/O time. Step 4: Response time is the time from submission until the first response is generated for the user, which might be the first output or screen update. Step 5: Select the option that states these definitions accurately and does not confuse them with hardware characteristics like CPU speed or memory size.

Verification / Alternative check:
As a check, imagine a scheduling algorithm that reduces waiting time and turnaround time for most jobs. You would expect response time to improve for interactive processes, and throughput might increase if more jobs finish per hour. These relationships only make sense if the metrics are defined as above. If throughput measured CPU clock speed or memory size, changing the algorithm would not affect it, which is clearly not the case. Therefore the option that keeps throughput as jobs per unit time and defines the other metrics in time terms is the correct one.

Why Other Options Are Wrong:
Option B is wrong because it confuses software performance metrics with hardware specifications. CPU clock speed, memory size, disk speed and network bandwidth are hardware characteristics, not the scheduling metrics asked in the question. Option C is wrong because throughput and waiting time are not identical and do not measure CPU run time in the same way; it also wrongly associates turnaround and response time purely with disk performance. Option D is wrong because throughput does not count context switches and all four metrics have well established definitions in operating system literature.

Common Pitfalls:
Students often mix up waiting time with turnaround time, forgetting that turnaround time includes CPU and I/O time in addition to waiting. Another common error is to equate response time with completion time, which ignores the importance of the first reply in interactive systems. Also, throughput is frequently misinterpreted as CPU utilization, even though the two are related but distinct. Keeping the definitions clear and practicing with small numeric examples helps to fix these metrics firmly in memory for exam questions.

Final Answer:
Thus, throughput is the number of jobs completed per unit time, turnaround time is total time from submission to completion, waiting time is total time spent in ready queues and response time is time from submission to first response.