The entire time taken for a process to run is most commonly called turnaround time. It is defined as the total time elapsed from the moment a process is submitted to a system until it is completed.
Turnaround time encompasses all stages of a process's journey, including:
- Waiting time: Time spent in the ready queue, waiting for the CPU.
- Execution time (or CPU time): The time the process actually spends using the CPU to execute instructions.
- I/O time: Time spent waiting for or performing input/output operations.
However, the term "execution time" is also used, sometimes interchangeably with turnaround time, but more precisely refers to the total time a computer takes to complete a task, including all associated overheads. In certain contexts, different, more specific metrics are used to measure process duration.
Detailed analysis of process timing metrics
To fully understand the duration of a process, it's essential to differentiate between various related timing metrics.
Turnaround time
This metric provides a high-level view of a process's total duration. In a multi-tasking operating system, a process does not run continuously from start to finish. Instead, it cycles through different states (ready, running, waiting) as the OS scheduler manages resource allocation. Turnaround time accounts for all these states, making it a critical metric for evaluating the efficiency of scheduling algorithms in a batch processing system.
Turnaround Time=Completion Time−Arrival TimeTurnaround Time equals Completion Time minus Arrival Time
TurnaroundTime=CompletionTime−ArrivalTime
Execution time vs. wall clock time vs. CPU time
These terms often cause confusion, but they measure distinct aspects of a process's running time.
- Execution time (or real time/wall clock time): This is the total elapsed time from the start of a process to its completion, as measured by a regular clock. It includes time spent executing, waiting for I/O, and waiting for CPU access. For example, if you start a program at 1:00 PM and it finishes at 1:05 PM, the wall clock time is five minutes. This is the most practical measure of performance from a user's perspective.
- CPU time (or process time): This is the total time the CPU was actively executing instructions for a specific process. It is further divided into:
- User CPU time: Time spent executing the user's code.
- System CPU time: Time the CPU spends on system calls or kernel operations on behalf of the process.
- The relationship:
- In a single-core system running only one process, wall clock time and CPU time would be nearly equal.
- In a modern multi-tasking or multi-core system, CPU time is often different from wall clock time.
- If a process is I/O-bound (waiting for network or disk), its wall clock time will be much longer than its CPU time.
- If a process is computationally intensive and runs on multiple cores, its total CPU time (sum of all cores) can exceed the wall clock time.
Waiting time
This is the amount of time a process spends in the ready queue, waiting for its turn to be allocated CPU time. It is influenced by the scheduling algorithm and the load on the system. High waiting times can lead to poor user experience, especially in interactive systems.
Response time
In interactive or time-sharing systems, this is a more relevant metric for user experience than turnaround time. Response time measures the duration from when a user submits a request until the first response is produced. A short response time is crucial for systems that need to feel responsive, even if the overall turnaround time for a complex task is long.
Context switching
This is the process of saving the state of one process and loading the state of another so that execution can be resumed later. It is an essential component of multitasking but is considered an overhead, as the system does no useful work during this time. The duration of context switching contributes to the overall wall clock time but is not part of the CPU time for any specific process.
Conclusion: A table of key process metrics
To summarize, here is a breakdown of the key process timing metrics:
| Metric | Definition | When to use |
|---|---|---|
| Turnaround Time | Total time from submission to completion. | Evaluating batch processing systems and scheduling algorithms. |
| Wall Clock Time | The actual elapsed time measured by a clock. | Assessing user-perceived performance. |
| CPU Time | Total time the CPU spends executing instructions for a process. | Measuring the computational intensity of a process. |
| Waiting Time | Time spent in the ready queue waiting for the CPU. | Analyzing the efficiency of scheduling algorithms under heavy load. |
| Response Time | Time from submission of request to the first output. | Evaluating interactive systems and user experience. |