● Covered by 1 source · 1 reportMedium impact1 neutral
JDK 27 Makes G1 the Default Garbage Collector, Introduces GC Code Refactorings
Key points
- G1 is now the default garbage collector in JDK 27 for all environments.
- Approximately half of the 350 GC changes are refactorings and cleanups.
- Refactorings include replacing volatile with Atomic<T> for shared variables.
- G1 no longer resizes the heap after Full GC based on MinHeapFreeRatio/MaxHeapFreeRatio.
G1 Becomes Default Garbage Collector
In OpenJDK 27, the G1 garbage collector has been designated as the default for all environments. This means that if no specific garbage collection algorithm is explicitly chosen via command-line options, G1 will be used. This decision eliminates the previous system of selecting different collectors based on environmental conditions, which was deemed a burden rather than an advantage.
Rationale for the Change
The shift to G1 as the universal default is attributed to its consistent improvements in throughput, footprint, and latency profile, which have brought it closer to the performance of the Serial GC in recent releases. While Serial GC can still be manually selected using the `-XX:+UseSerialGC` option for specific application profiles where it might perform better, G1 is now considered the optimal starting point for most applications.
Extensive Code Refactoring and Cleanups
The JDK 27 release includes approximately 350 changes to the garbage collection subcomponent, with about half of these being refactorings and code cleanups. A significant refactoring involved replacing the use of `volatile` and `AtomicAccess` methods for cross-thread shared variables with `Atomic<T>`, making their usage more explicit. Other notable improvements include cleanups to the G1 state machine and general code enhancements.
Bug Fixes and New Behaviors
Roughly 35% of the changes address bug fixes and general improvements to correctness and robustness within the garbage collector code. The remaining changes introduce new or revised behaviors and features for HotSpot's stop-the-world collectors. Infrastructure work for JEP 401: Value Objects (Preview), such as adapting object iterators and eager reclaim, was also integrated.
G1 Heap Resizing Adjustment
Beyond the default status, G1 also received an update where it no longer resizes the heap after a Full GC based on the `-XX:MinHeapFreeRatio` and `-XX:MaxHeapFreeRatio` parameters. This change streamlines G1's heap management behavior.
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Reporting from
OpenJDK 27 will make the G1 garbage collector the default for all environments, removing the previous conditional defaults. This change reflects G1's improved performance and simplifies garbage collector selection for developers.