Load testing calculation involves determining the number of virtual users needed to simulate a desired load on a system. The fundamental relationship connects virtual users, requests per second (RPS), and latency.
The core calculation uses the following formula:
Virtual Users = (RPS) * (Latency in Seconds)
Here's a breakdown and explanation of the formula and its application:
Understanding the Components
- Virtual Users: The number of simulated users concurrently interacting with the system. This is what you're trying to determine.
- RPS (Requests Per Second): The target throughput of requests the system needs to handle per second. This is often a business requirement or a performance goal.
- Latency (in Seconds): The average time it takes for the system to respond to a request. This is a critical performance indicator that you'll measure during testing, but you'll need an estimated or target latency value before you start testing in order to properly configure your load test.
Calculation Example
Let's say you need to simulate 100,000 requests per second (RPS) on an application. You estimate or expect that the application's latency will be 20 milliseconds (ms). To use the formula, you first need to convert the latency to seconds:
20 ms = 0.02 seconds
Now, apply the formula:
Virtual Users = (100,000 RPS) * (0.02 seconds) = 2,000 Virtual Users
Therefore, you should configure your load test with 2,000 virtual users to simulate 100,000 RPS, given an estimated latency of 20 milliseconds.
Practical Considerations and Additional Factors
- Ramp-up Time: Consider the time it takes to increase the number of virtual users to the target level. A gradual ramp-up might be more realistic than an instantaneous jump.
- Test Duration: Determine how long the load test should run. Sufficient duration is needed to observe steady-state performance and identify potential bottlenecks.
- Think Time: Simulate realistic user behavior by including "think time" between requests, which represents the time a user spends reading content or filling out forms. Too little think time will artificially inflate RPS and require a higher number of VUs than is truly representative of actual users.
- Resource Monitoring: Monitor server resources (CPU, memory, disk I/O, network I/O) during the load test to identify bottlenecks and areas for optimization. This data is critical for understanding why your application is behaving a certain way.
- Test Environment: Ensure the test environment closely mirrors the production environment to get accurate results.
- Different Scenarios: Consider testing different user scenarios and mixes to understand the system's performance under various conditions. For example, test a user scenario where the user is searching for data vs. creating new data.
Iterative Process
Load testing is rarely a one-time event. It's an iterative process. You'll likely:
- Estimate latency and determine the required virtual users.
- Run the load test.
- Measure the actual latency and RPS.
- Adjust the number of virtual users based on the observed results to fine-tune the load.
- Repeat steps 2-4 as needed.
By following these steps, you can effectively calculate and execute load tests to ensure your system meets its performance requirements.
