How to evaluate "&" vs "%" correctly using JMH

Question

So, here is the simple thing I'm trying to test, that a mod or AND operation is faster (assuming it is equal to two) - this is what hashMap does internally. Is this a correctly written "test"? I have to admit that the insides of jmh and getting the right micro-test after passing all the samples (within the 3rd time, I think) is a rather difficult task. :)

@State(Scope.Thread) @BenchmarkMode(org.openjdk.jmh.annotations.Mode.AverageTime) @OutputTimeUnit(TimeUnit.NANOSECONDS) public class MeasureSpeedModuleVsAnd { public static void main(String[] args) throws Exception { Options opt = new OptionsBuilder() .include(MeasureSpeedModuleVsAnd.class.getSimpleName()) .forks(1) .warmupIterations(1) .measurementIterations(5) .warmupTime(TimeValue.seconds(2)) .build(); new Runner(opt).run(); } @Param({ "16", "32", "256", "1048576" /* 2 power of 10 */ }) public int number_of_buckets; @Param({ "345984", "123456", "111", "98653" }) public int hashcode; @Benchmark public int benchamark_modulo() { return hashcode % number_of_buckets; } @Benchmark public int benchmark_and() { return (number_of_buckets - 1) & hashcode; } } 

Answer 1

This is described in detail in this blog post: http://psy-lob-saw.blogspot.co.za/2014/11/the-mythical-modulo-mask.html

Your test does not work (comparing what seems to be inappropriate values) because you are comparing (non_final_field and constant) with (constant% non_final_field). Replace (non_final_field1% non_final_field2) and (non_final_field1 and (non_final_field2-1)), where non_final_field2 is a power of 2.

In the context of a HashMap, a value is used to read from an array, and a blog post also covers the consequences of this side.