org.openjdk.jmh.samples.JMHSample_08_DeadCode Maven / Gradle / Ivy

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package org.openjdk.jmh.samples;

import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.GenerateMicroBenchmark;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;

import java.util.concurrent.TimeUnit;

@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class JMHSample_08_DeadCode {

    /*
     * The culprit of many benchmarks is the dead-code elimination: compilers are smart
     * enough to deduce some computations are redundant, and eliminate them completely.
     * If that eliminated part was our benchmarked code, we are in trouble.
     *
     * Fortunately, JMH provides the essential infrastructure to fight this where appropriate:
     * returning the result of the computation will ask JMH to deal with the result to limit
     * the dead-code elimination.
     */

    private double x = Math.PI;

    @GenerateMicroBenchmark
    public void baseline() {
        // do nothing, this is a baseline
    }

    @GenerateMicroBenchmark
    public void measureWrong() {
        // This is wrong: result is not used, and the entire computation is optimized out.
        Math.log(x);
    }

    @GenerateMicroBenchmark
    public double measureRight() {
        // This is correct: the result is being used.
        return Math.log(x);
    }

    /*
     * ============================== HOW TO RUN THIS TEST: ====================================
     *
     * You can see the unrealistically fast calculation in with measureWrong(),
     * while realistic measurement with measureRight().
     *
     * You can run this test:
     *
     * a) Via the command line:
     *    $ mvn clean install
     *    $ java -jar target/microbenchmarks.jar ".*JMHSample_08.*" -wi 5 -i 5 -f 1
     *    (we requested 5 warmup/measurement iterations, single fork)
     *
     * b) Via the Java API:
     */

    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder()
                .include(".*" + JMHSample_08_DeadCode.class.getSimpleName() + ".*")
                .warmupIterations(5)
                .measurementIterations(5)
                .forks(1)
                .build();

        new Runner(opt).run();
    }

}