关于JMeter并发与恒定吞吐量的技术问题咨询

Hey Ajay, great question! Let me break down how JMeter's Constant Throughput Timer interacts with thread count, especially when your API has zero response delay.

Your initial understanding isn't fully accurate—here's what actually happens in practice:

• The Constant Throughput Timer’s core purpose is to control the overall request rate of your test (or the specific scope you configure it for), rather than letting threads burst first and then throttle back.
• When you set 5 threads and the API has no delay, JMeter will try to keep those 5 threads active, but the timer will dynamically insert pauses between requests to ensure the total number of requests per unit time matches your configured throughput.

Let’s use concrete examples to make this clear:

1. If you set a throughput of 60 requests per minute (1 request/second): Even though all 5 threads are ready to send requests immediately, the timer will enforce a 1-second gap between each request. You won’t see a burst of 5 concurrent requests—instead, requests will be evenly spaced to hit the 1/sec rate.
2. If you set a throughput of 300 requests per minute (5 requests/second): Each of the 5 threads will send one request per second, resulting in exactly 5 concurrent requests (one from each thread) every second. This feels like steady concurrency, but it’s still the timer coordinating the rate to hit your target.
3. If you set a throughput higher than what 5 threads can handle (e.g., 1000 requests per minute with zero delay): The 5 threads will run at full capacity, sending requests as fast as possible, and the timer won’t need to add any pauses—since the actual rate won’t exceed your target.

A key detail to note: The Constant Throughput Timer calculates required delays based on the elapsed time since the test started and the number of requests already sent. It doesn’t "save up" requests to burst later; it adjusts dynamically to maintain the desired rate from the test’s start.

To sum up: You won’t get an initial burst of 5 requests followed by throttling unless your throughput target is so high that 5 threads can’t reach it immediately. In most cases, the timer will regulate the rate from the very beginning, aligning request sending with your configured throughput.

内容的提问来源于stack exchange，提问作者Ajay