I have setup a jmeter script with constant runtime set in runtime controller, infinite loop in loop controller and a constant delay between threads in'constant timer'. How can I perform tuning using this setup? Is there a correlation between 'no of threads', 'rampup time' and 'delay' that should be kept in mind while trying different combinations of these values for performance testing?
Number of threads is basically the number of users you will be simulating. Each JMeter thread (or virtual user) must represent a real user using your application so treat it this way. If you have a requirement that the application must support 1000 concurrent users - stick to this number as the baseline for your testing. With regards to "how much load will my N JMeter users generate" - it depends on several factors like nature of your test, server response time, timers, Ramp-up is the time for JMeter to kick off the virtual users from point 1. Unless you're doing spike testing you should be increasing the load gradually as if you release all the users right away you will get much less information and in case of gradual increasing the load you will be able to correlate it with increasing response time and decreasing throughput, number of errors, etc. Moreover it will allow to "wamp up" application under test and it will be more ready for the stressDelay is time the virtual user is "thinking" between operations. Real users don't hammer application non-stop, they need some time to "think" before making the next step. Depending on what the user is "doing" the think time might be different so I would recommend going for Uniform Random Timer instead of the "Constant" one.