Does Java's Garbage Collector Always Return Memory to the Operating System?

Does GC Return Memory to the OS?

The garbage collector (GC) in Java aims to reclaim unused memory. Does this reclaimed memory return to the operating system (OS)?

HotSpot JVM's Memory Management

The HotSpot JVM, which implements Java's runtime, does release memory back to the OS. However, this process is selective and not always immediate. The JVM assumes that the unused memory will likely be needed in the future, reducing the overhead of resizing the heap.

Factors Influencing Memory Release

The specific GC algorithm used and the JVM version impact the ability to release memory. Some collectors, like EpsilonGC, may never support it. Others, like Shenandoah, OpenJ9 VM, and ZGC (in Java 13 onwards), offer explicit options to enable prompt memory release.

Options for Aggressive Memory Release

In JDK 8 and earlier, explicit options for memory reclamation are limited. However, the following measures can encourage more aggressive GC behavior:

• Setting -XX:GCTimeRatio=19 -XX:MinHeapFreeRatio=20 -XX:MaxHeapFreeRatio=30
• Enabling concurrent collections with -XX:InitiatingHeapOccupancyPercent=N (with N set low) for concurrent collectors

In JDK 9, -XX:-ShrinkHeapInSteps enables more aggressive shrinking. In JDK 12, -XX:G1PeriodicGCInterval promotes prompt memory release for G1GC.

Logging and Monitoring

To verify memory shrinking or diagnose reasons for lack of it, use GC logging with -XX: PrintAdaptiveSizePolicy (deprecated in JDK 9, replaced with -Xlog:gc ergo). This logging provides insights into the JVM's adaptive size policy decisions, which can influence memory release behavior.

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