org.openjdk.jmh.samples.JMHSample_11_Loops 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_11_Loops {
/*
* It would be tempting for users to do loops within the benchmarked method.
* (This is the bad thing Caliper taught everyone). These tests explain why
* this is a bad idea.
*
* Looping is done in the hope of minimizing the overhead of calling the
* test method, by doing the operations inside the loop instead of inside
* the method call. Don't buy this argument; you will see there is more
* magic happening when we allow optimizers to merge the loop iterations.
*/
/*
* Suppose we want to measure how much it takes to sum two integers:
*/
int x = 1;
int y = 2;
/*
* This is what you do with JMH.
*/
@Benchmark
public int measureRight() {
return (x + y);
}
/*
* The following tests emulate the naive looping.
* This is the Caliper-style benchmark.
*/
private int reps(int reps) {
int s = 0;
for (int i = 0; i < reps; i++) {
s += (x + y);
}
return s;
}
/*
* We would like to measure this with different repetitions count.
* Special annotation is used to get the individual operation cost.
*/
@Benchmark
@OperationsPerInvocation(1)
public int measureWrong_1() {
return reps(1);
}
@Benchmark
@OperationsPerInvocation(10)
public int measureWrong_10() {
return reps(10);
}
@Benchmark
@OperationsPerInvocation(100)
public int measureWrong_100() {
return reps(100);
}
@Benchmark
@OperationsPerInvocation(1_000)
public int measureWrong_1000() {
return reps(1_000);
}
@Benchmark
@OperationsPerInvocation(10_000)
public int measureWrong_10000() {
return reps(10_000);
}
@Benchmark
@OperationsPerInvocation(100_000)
public int measureWrong_100000() {
return reps(100_000);
}
/*
* ============================== HOW TO RUN THIS TEST: ====================================
*
* You might notice the larger the repetitions count, the lower the "perceived"
* cost of the operation being measured. Up to the point we do each addition with 1/20 ns,
* well beyond what hardware can actually do.
*
* This happens because the loop is heavily unrolled/pipelined, and the operation
* to be measured is hoisted from the loop. Morale: don't overuse loops, rely on JMH
* to get the measurement right.
*
* You can run this test:
*
* a) Via the command line:
* $ mvn clean install
* $ java -jar target/benchmarks.jar JMHSample_11 -f 1
* (we requested single fork; there are also other options, see -h)
*
* 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_11_Loops.class.getSimpleName())
.forks(1)
.build();
new Runner(opt).run();
}
}