Unravel › Java concepts › JVM and performance

At a glance

1. 1long t0 = System.nanoTime(); work(); long ms = (System.nanoTime() - t0) / 1_000_000; is fine to find out where a request spends its time in a log line, but not to compare two implementations
2. 2JMH: annotate a method with @Benchmark, state in a @State class, and use @Warmup(iterations = 5), @Measurement(iterations = 5), @Fork(2)Return the result or give it to a Blackhole so the JIT cannot discard the work.
3. 3Reports give average time, error and unitsCompare means with their error bars; a 3 percent difference inside a 10 percent error is noise.
4. 4For real systems measure with a profiler (JFR, async-profiler) under realistic load, because microbenchmarks do not include GC, contention or I/O

• Typical traps: constant folding (inputs known at compile time), dead-code elimination (unused result), loop unrolling that is not representative, a cold JIT, running on a laptop with power saving, and measuring one thread when production has many
• Optimise after measuring where the time is; an algorithmic improvement (see Big O) usually beats micro-tuning
• HotSpot starts interpreting bytecode, compiles hot methods with C1 and then C2, and may deoptimise later

import org.openjdk.jmh.annotations.*;import java.util.concurrent.TimeUnit;@State(Scope.Thread)@BenchmarkMode(Mode.AverageTime)@OutputTimeUnit(TimeUnit.NANOSECONDS)@Warmup(iterations = 3)@Measurement(iterations = 5)@Fork(1)public class SumBench {  int[] data = new int[1000];  @Benchmark  public long loop() {    long s = 0;    for (int v : data) s += v;    return s;  }}

1. line 3 uses State, the core of Measuring performance and JMH
2. line 4 uses Benchmark, the core of Measuring performance and JMH
3. line 5 uses OutputTimeUnit, the core of Measuring performance and JMH
4. line 6 uses Warmup, the core of Measuring performance and JMH
5. line 7 uses Measurement, the core of Measuring performance and JMH
6. line 8 uses Fork, the core of Measuring performance and JMH

How Unravel recognises it

When you paste code, Unravel looks for these signs of measuring performance and jmh. Spotting them yourself is a good habit when you read unfamiliar code:

• Annotations: @Benchmark @State @Warmup @Measurement @Fork @BenchmarkMode @OutputTimeUnit @Setup @Param
• Types: Blackhole StopWatch Stopwatch OptionsBuilder
• Method calls: System.nanoTime() System.currentTimeMillis() Instant.now()

Part of a learning path

• The JVM and the environment · previous: Class loading and the class path · next: Method handles, VarHandle and native code

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