What does CompileThreshold, Tier2CompileThreshold, Tier3CompileThreshold and Tier4CompileThreshold control?

Question

HotSpot\'s tiered compilation uses the interpreter until a threshold of invocations (for methods) or iterations (for loops) triggers a client compilation with self-profiling

Answer 1

The comments in advancedThresholdPolicy.hpp discuss the different compiler tiers and the thresholds. See that file for a deeper discussion.

The system supports 5 execution levels:

• Tier 0 - interpreter
• Tier 1 - C1 with full optimization (no profiling)
• Tier 2 - C1 with invocation and backedge counters
• Tier 3 - C1 with full profiling (level 2 + MDO)
• Tier 4 - C2

C1 is the client compiler. C2 is the server compiler.

In the common case, the compilation goes: 0 → 3 → 4. Atypical cases are used based on C1 and C2 queue lengths. Tier 2 is used when the C2 queue length is too long so that the method can execute about 30% faster until the C2 can process the profiling information. If the method is determined to be trivial, then it is compiled with Tier 1 since it will produce the same code as Tier 4.

Thresholds are dynamically adjusted based on the length of the C1 and C2 queues.