JMeter: Difference between using Number of Threads and Constant Throughput Timer

Question

What's the difference between Number of Threads and the Constant Throughput Timer? Before I heard of the Constant Throughput Timer, I thought that if I wanted to simulate 21 requests per minute , the following configuration was enough :

but now that I found this post, I'm not so sure.

This is my new configuration now that I make use of the Constant Throughput Timer :

1. Does the Constant Throughput Timer override the parameters specify in Thread Group (Number of threads,Ramp-up period, etc) ?
2. Why even making use of the Constant Throughput Timer I sometimes get a number of requests higher than the value specify in the Target Throughput (21 in the above image)
3. If a indicate a value of 21 in the Target Throughput, isn't this constant supposed to guarantee a maximum value of 21 requests? If so why do I sometimes get more requests than indicated.

All your comments and ideas will be appreciated.

P.S. I'm also using a Runtime controller.

Answer 1

No, it regulates the number of requests going to & response received from the server per minute.

As stated in the Jmeter user guide "the throughput value should not be changed too often during a test - it will take a while for the new value to take effect."

http://jmeter.apache.org/usermanual/component_reference.html#Constant_Throughput_Timer

The issue you have shown above will always happen in the first minute of the test run.

hope this will help.