JIT tuning

Learn JIT flags and tiered compilation settings.

JIT compilation

The JIT (Just-In-Time) compiler turns frequently called hot methods into machine code. HotSpot uses two compilers: C1 compiles fast (little optimisation), C2 does aggressive optimisation. Tiered compilation (-XX:+TieredCompilation) uses C1 first, then C2.

The compile threshold is set with -XX:CompileThreshold; a method is compiled once it is called this many times. -XX:+PrintCompilation shows what gets compiled.

JIT tuning

JIT tuning is rarely needed; the default tiered compilation is good for most workloads. Still, -XX:+PrintCompilation shows what is compiled and -XX:+UnlockDiagnosticVMOptions reveals detailed flags.

Premature tuning should be avoided; first find the bottleneck with JFR and JMH. The aggressive optimisation of C2 sometimes gives surprising speedups; that is why microbenchmarking uses JMH.

Exercise Write the JitConfig class

Define class JitConfig with a static String describe(int threshold) method that returns "CompileThreshold=" + threshold + " Tiered=true". In main print System.out.println(describe(10000));.

Show example solution

class JitConfig {
    static String describe(int threshold) {
        return "CompileThreshold=" + threshold + " Tiered=true";
    }
    public static void main(String[] args) {
        System.out.println(describe(10000));
    }
}

Test yourself

Complete the tiered compilation flag

-XX:+____ -XX:CompileThreshold=10000

Show answer

-XX:+TieredCompilation -XX:CompileThreshold=10000

Match the JIT concepts

C1

Fast compiler
C2

Optimising compiler
CompileThreshold

Compile threshold
TieredCompilation

Tiered compilation
What does the JIT do?
- It compiles source code
- It creates threads
- It collects garbage
- It turns hot methods into machine code
Show answer

It turns hot methods into machine code — The JIT compiles frequently called methods into machine code.

Next lesson: Summary - Performance and GC

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