HotSpot JIT Tiered Compilation & Inlining

The HotSpot JVM employs a multi-tiered JIT compilation strategy to balance fast application startup with peak execution throughput.

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Compilation Tier Progression

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Method Inlining Threshold Rules

HotSpot makes inlining decisions based on compiled bytecode size:

Inlining Setting	Bytecode Threshold	Decision Behavior
-XX:MaxInlineSize=35	$\le$ 35 bytes	Always Inlined regardless of execution frequency.
-XX:FreqInlineSize=325	$\le$ 325 bytes	Inlined if the method callsite is marked hot by C1 profiling counters.
Large Methods	$>$ 325 bytes	Inlining Rejected. Invocation incurs standard call dispatch overhead.

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Experiment Output Example

./scripts/start-helix.sh experiment --name jit --output json

{
  "experiment" : "JIT",
  "status" : "SUCCESS",
  "scenario" : "HotSpot Tiered Compilation Progression",
  "highestTierAchieved" : "Tier 4 (C2)",
  "inliningDecision" : "Inlined (size <= 35B)"
}