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Table of contents
What is TAT? 
Example of TAT
What's the Use of TAT
What is WAT? 
Example of WAT
What's the Use of WAT
Difference Table of TAT vs WAT
Frequently Asked Questions
How can TAT and WAT be optimized in an operating system?
Can a process have a high TAT but low WAT?
Are TAT and WAT relevant in modern, multi-core processors?
Last Updated: Mar 27, 2024


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Turnaround time (TAT) and Wait Time (WAT) are fundamental concepts in the realm of operating systems (OS) that play a crucial role in process management and scheduling. Understanding these metrics is vital for both OS designers and users to evaluate and improve the performance of an operating system. In this article, we will delve deep into what TAT and WAT are, their significance, and how they differ. 


This exploration will provide theoretical and practical insights through examples, making it beneficial for students and professionals alike.

What is TAT? 

Turnaround Time (TAT) is a critical metric in operating systems that measures the total time a process takes to complete after it has been submitted. TAT is calculated from when a task enters the system (submission time) to when it completes its execution (completion time). This duration includes all the phases of a process lifecycle — waiting, execution, and any I/O operations.

TAT is a comprehensive measure, encompassing all the delays a process might encounter. It reflects the effectiveness of the scheduling algorithms and is crucial in environments where time is of the essence, like real-time systems.

To understand TAT better, let's consider an example.

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Example of TAT

Suppose a process P1 enters a system at time 0 and completes its execution at 20 units. During this period, it spends 5 units of time waiting in the queue and 15 units being processed. The TAT for P1 would be:

TAT=20−0=20 units

This example demonstrates that TAT not only accounts for the actual processing time but also includes the waiting time in the system.

What's the Use of TAT

TAT is instrumental in several aspects:

  • Performance Measurement: It helps in assessing the efficiency of scheduling algorithms. A lower TAT is often desirable as it indicates quicker processing of tasks.
  • System Evaluation: TAT is used to evaluate the overall performance of an operating system. It is particularly important in time-sensitive environments like stock trading systems or real-time data processing.
  • User Experience: For end-users, TAT is a direct measure of how long they have to wait for a task to be completed, impacting the user experience.

What is WAT? 

Wait Time (WAT) is another pivotal metric in operating systems, representing the amount of time a process spends waiting in the ready queue before it gets its chance to execute. Unlike TAT, which encompasses the entire lifecycle of a process, WAT focuses solely on the duration a process remains idle before its execution begins.

WAT is a direct indicator of the efficiency of CPU scheduling. A high WAT can imply potential inefficiencies in the scheduling algorithm, leading to longer queues and increased waiting times for processes.

To better grasp WAT, let's look at an illustrative example.

Example of WAT

Consider a process P2 that arrives in the ready queue at time 5 units. It starts execution at time 15 units. The WAT for P2 would be:

WAT=15−5=10 units

This example shows that WAT is exclusively the delay experienced by the process in the ready queue, not accounting for the actual execution or I/O operations.

What's the Use of WAT

The importance of WAT in operating systems is manifold:

  • Scheduling Efficiency: It helps in evaluating the fairness and efficiency of the CPU scheduling algorithm. A lower WAT suggests that processes are being allocated CPU time without significant delays.
  • Resource Utilization: By analyzing WAT, system administrators can identify bottlenecks and optimize resource allocation for better throughput.
  • Balancing Load: In multi-user or multi-tasking environments, WAT is crucial for balancing the load among various processes, ensuring that no single process is starved of CPU time.

Difference Table of TAT vs WAT

Understanding the distinctions between Turnaround Time (TAT) and Wait Time (WAT) is crucial for comprehending their roles in process scheduling within operating systems. The following table outlines the key differences between these two metrics:

Aspect Turnaround Time (TAT) Wait Time (WAT)
Definition Total time taken from the submission to the completion of a process. Time a process spends waiting in the queue before execution.
Inclusion Includes waiting time, execution time, and I/O time. Includes only the waiting time in the ready queue.
Measure of Overall time efficiency of the system. Efficiency of CPU scheduling and process prioritization.
Importance Indicates the total time taken for process completion. Indicates the responsiveness of the system to a process.
Impact on Performance Lower TAT indicates efficient process handling and system performance. Higher WAT can imply inefficiencies in scheduling and potential for optimization.
Use Case Important in time-critical systems for overall performance analysis. Critical in multi-tasking environments to ensure fair CPU allocation.

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Frequently Asked Questions

How can TAT and WAT be optimized in an operating system?

Optimizing TAT and WAT often involves fine-tuning the scheduling algorithm, ensuring fair and efficient allocation of CPU time, and possibly enhancing resource management to reduce bottlenecks.

Can a process have a high TAT but low WAT?

Yes, this is possible if a process spends a significant amount of time in execution or I/O operations, leading to a high TAT despite a low waiting time.

Are TAT and WAT relevant in modern, multi-core processors?

Absolutely. Even in multi-core environments, managing process queues and CPU allocation efficiently remains critical, making TAT and WAT relevant metrics for performance evaluation.


In conclusion, Turnaround Time (TAT) and Wait Time (WAT) are essential metrics in the field of operating systems, providing insights into process management and scheduling efficiency. TAT offers a broad perspective on the total time a process spends in the system, while WAT zeroes in on the waiting period before execution. Understanding these concepts is crucial for anyone involved in operating system design, implementation, or optimization, as they directly impact system performance and user experience.

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