Monitoring is an integral part of the performance test execution phase. It is performed simultaneously with the application of the test load to observe the System Under Test's (SUT) real-time behavior. This concurrent monitoring allows testers to correlate the generated load with key performance indicators (KPIs) such as response time, throughput, and resource utilization (e.g., CPU, memory). This immediate feedback is crucial for identifying performance bottlenecks and understanding how the system behaves under specific stress conditions as they occur.

A. When testing is concluded: This is incorrect. Monitoring after the test is for post-test analysis of logs and collected data, not the active monitoring process during the load test itself.

B. Before testing execution: This is incorrect. While establishing a baseline by monitoring the system at rest before the test is a common and valuable practice, the primary monitoring activity occurs during the test to see the effect of the load.

C. Immediately after test execution: This is incorrect. This timing is associated with analyzing results and observing system recovery, not with monitoring the system's performance under the applied load.

1. ISTQB® Certified Tester Performance Testing Syllabus v2.0 (2018): Section 4.3.1

"Performance Test Execution

" states: "During test execution

the test environment and the SUT are monitored to check for any unexpected behavior and to ensure that the test environment itself is not the cause of any performance issues." This explicitly places monitoring during the execution phase. (Source: ISTQB® Official Syllabus).

2. Jain

R. (1991). The Art of Computer Systems Performance Analysis: Techniques for Experimental Design

Measurement

Simulation

and Modeling. Wiley. Chapter 3

"Measurement Techniques and Tools

" discusses that monitoring tools are used to observe system behavior while workloads are running to collect performance data. This foundational text establishes the principle of concurrent measurement and load application. (Page 35).

3. Weyuker

E. J.

& Vokolos

F. I. (2000). Experience with performance testing of software systems: issues

an approach

and case study. IEEE Transactions on Software Engineering

26(12)

1147-1156. The paper's methodology describes the test execution phase where "the system is stressed by a driver that simulates a specified number of users... While the system is being stressed

performance data is collected." This confirms that data collection (monitoring) is simultaneous with load generation. (Section IV. A. Test Execution).