What's the cnt column in the jmh results?

Question

In my jmh class, I'm using

@BenchmarkMode(Mode.SampleTime)
@Measurement(iterations = 10)
@Threads(value = 10)

at the class level.

And in the results, for the cnt column, I see 132, for one method and 155 for another. I was assuming cnt might mean number of executions but those numbers don't match what I'm expecting.

Shouldn't number of executions be 10 (threads) * 10 (iterations) = 100?

Answer 1

Frankly, I do not know how cnt is calculated when benchmark mode is Mode.SampleTime, but I want to clarify a couple of things.

First of all, benchmarking with JMH is concurrent in nature. The @Threads annotation specifies that ten threads should be allocated to run your benchmark (default is 4). If you do not specify @Threads, it will allocate only one thread for benchmarking.

Secondly, if you change your mode to Mode.Throughput and add @Fork(value = 5) annotation, you will see cnt column with value 50. Because your benchmark will be executed in 5 forks and each fork will have 10 iterations. Simply, cnt is forkNumber * measurementIterations for this case.

Finally, this calculation changes when mode is Mode.SampleTime or when you use @Param annotation (cnt = numOfParams * forkNumber * measurementIterations). For example, if you run different benchmark with same annotations (Mode.SampleTime), you'll see different numbers on cnt column.

I hope it helps!