Integrating Profile Caching into the HotSpot Multi-Tier Compilation System
Zoltán Majó, Tobias Hartmann, Marcel Mohler, Thomas R. Gross · 2017
The Java®HotSpot Virtual Machine includes a multi-tier compilation system that may invoke a compiler at any time. Lower tiers instrument the program to gather information for the highly optimizing compiler at the top tier, and this compiler bases its optimizations on these profiles. But if the assumptions made by the top-tier compiler are proven wrong (e.g., because the profile does not cover all execution paths), the method is deoptimized: the code generated for the method is discarded and the method is then executed at Tier 0 again. Eventually, after profile information has been gathered, the method is recompiled at the top tier again (this time with less-optimistic assumptions). Users of the system experience such deoptimization cycles (discard, profile, compile) as performance fluctuations and potentially as variations in the system's responsiveness. Unpredictable performance however is problematic in many time-critical environments even if the system is not a hard real-time system.