May 2020
Beginner
267 pages
7h 37m
English
Content preview from 97 Things Every Java Programmer Should Know
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Chapter 6. Benchmarking Is Hard—JMH Helps
Michael Hunger
Benchmarking on the JVM, especially microbenchmarking, is hard. It’s not enough to throw a nanosecond measurement around a call or loop and be done. You have to take into account warm-up, HotSpot compilation, code optimizations like inlining and dead code elimination, multithreading, consistency of measurement, and more.
Fortunately, Aleksey Shipilëv, the author of many great JVM tools, contributed JMH, the Java Microbenchmarking Harness, to the OpenJDK. It consists of a small library and a build system plug-in. The library provides annotations and utilities to declare your benchmarks as annotated Java classes and methods, including a BlackHole class to consume generated values to avoid code elimination. The library also offers correct state handling in the presence of multithreading.
The build system plug-in generates a JAR with the relevant infrastructure code for running and measuring the tests correctly. That includes dedicated warm-up phases, proper multithreading, running multiple forks and averaging across them, and much more.
The tool also outputs important advice on how to use the gathered data and limitations thereof. Here is an example for measuring the impact of presizing collections:
public class MyBenchmark { static final int COUNT = 10000; @Benchmark public List<Boolean> testFillEmptyList() { List<Boolean> ...