here's a useful link: http://jmeter.512774.n5.nabble.com/Delayed-thread-creation-was-OnDemand-ThreadGroup-td5714355.html
focus when he says: “When this is selected, threads in the thread group are only created and are started when they should be started. Start delay (if any) and slew delay are now executed until the stream data is created.
This means that memory requirements are proportional to the number of simultaneous active threads, and not to the total number of threads. Provided that the active account is not too large, it should be possible to run tests with a very large maximum number of threads.
If “Delayed thread creation” is not selected, JMeter behaves as before, that is, all thread flows are created at the beginning of the test. "
Hope this helps!
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