org.openjdk.jmh.samples.JMHSample_12_Forking Maven / Gradle / Ivy

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package org.openjdk.jmh.samples;

import org.openjdk.jmh.annotations.*;
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_12_Forking {

    /*
     * JVMs are notoriously good at profile-guided optimizations. This is bad
     * for benchmarks, because different tests can mix their profiles together,
     * and then render the "uniformly bad" code for every test. Forking (running
     * in a separate process) each test can help to evade this issue.
     *
     * JMH will fork the tests by default.
     */

    /*
     * Suppose we have this simple counter interface, and two implementations.
     * Even though those are semantically the same, from the JVM standpoint,
     * those are distinct classes.
     */

    public interface Counter {
        int inc();
    }

    public static class Counter1 implements Counter {
        private int x;

        @Override
        public int inc() {
            return x++;
        }
    }

    public static class Counter2 implements Counter {
        private int x;

        @Override
        public int inc() {
            return x++;
        }
    }

    /*
     * And this is how we measure it.
     * Note this is susceptible for same issue with loops we mention in previous examples.
     */

    public int measure(Counter c) {
        int s = 0;
        for (int i = 0; i < 10; i++) {
            s += c.inc();
        }
        return s;
    }

    /*
     * These are two counters.
     */
    Counter c1 = new Counter1();
    Counter c2 = new Counter2();

    /*
     * We first measure the Counter1 alone...
     * Fork(0) helps to run in the same JVM.
     */

    @Benchmark
    @Fork(0)
    public int measure_1_c1() {
        return measure(c1);
    }

    /*
     * Then Counter2...
     */

    @Benchmark
    @Fork(0)
    public int measure_2_c2() {
        return measure(c2);
    }

    /*
     * Then Counter1 again...
     */

    @Benchmark
    @Fork(0)
    public int measure_3_c1_again() {
        return measure(c1);
    }

    /*
     * These two tests have explicit @Fork annotation.
     * JMH takes this annotation as the request to run the test in the forked JVM.
     * It's even simpler to force this behavior for all the tests via the command
     * line option "-f". The forking is default, but we still use the annotation
     * for the consistency.
     *
     * This is the test for Counter1.
     */

    @Benchmark
    @Fork(1)
    public int measure_4_forked_c1() {
        return measure(c1);
    }

    /*
     * ...and this is the test for Counter2.
     */

    @Benchmark
    @Fork(1)
    public int measure_5_forked_c2() {
        return measure(c2);
    }

    /*
     * ============================== HOW TO RUN THIS TEST: ====================================
     *
     * Note that C1 is faster, C2 is slower, but the C1 is slow again! This is because
     * the profiles for C1 and C2 had merged together. Notice how flawless the measurement
     * is for forked runs.
     *
     * You can run this test:
     *
     * a) Via the command line:
     *    $ mvn clean install
     *    $ java -jar target/benchmarks.jar JMHSample_12
     *
     * b) Via the Java API:
     *    (see the JMH homepage for possible caveats when running from IDE:
     *      http://openjdk.java.net/projects/code-tools/jmh/)
     */

    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder()
                .include(JMHSample_12_Forking.class.getSimpleName())
                .build();

        new Runner(opt).run();
    }

}