How does the JVM decided to JIT-compile a method (categorize a method as “hot”)?

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HotSpot compilation policy is rather complex, especially for Tiered Compilation, which is on by default in Java 8. It’s neither a number of executions, nor a matter of CompileThreshold parameter.

The best explanation (apparently, the only reasonable explanation) can be found in HotSpot sources, see advancedThresholdPolicy.hpp.

I’ll summarize the main points of this advanced compilation policy:

  • Execution starts at tier 0 (interpreter).
  • The main triggers for compilation are
    1. method invocation counter i;
    2. backedge counter b. Backward branches typically denote a loop in the code.

  • Every time counters reach certain frequency value (TierXInvokeNotifyFreqLog, TierXBackedgeNotifyFreqLog), a compilation policy is called to decide what to do next with currently running method. Depending on the values of i, b and current load of C1 and C2 compiler threads it can be decided to

    • continue execution in interpreter;
    • start profiling in interpreter;
    • compile method with C1 at tier 3 with full profile data required for futher recompilation;
    • compile method with C1 at tier 2 with no profile but with possibility to recompile (unlikely);
    • finally compile method with C1 at tier 1 with no profile or counters (also unlikely).

    Key parameters here are TierXInvocationThreshold and TierXBackEdgeThreshold. Thresholds can be dynamically adjusted for a given method depending on the length of compilation queue.

  • Compilation queue is not FIFO, but rather a priority queue.

  • C1-compiled code with profile data (tier 3) behave similarly, except that thresholds for switching to the next level (C2, tier 4) are much bigger. E.g. an interpreted method can be compiled at tier 3 after about 200 invocations, while C1-compiled method is subject for recompilation at tier 4 after 5000+ invocations.

  • A special policy is used for method inlining. Tiny methods can be inlined into the caller even if they are not “hot”. A bit larger methods can be inlined only if they are invoked frequently (InlineFrequencyRatio, InlineFrequencyCount).

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