Does Java Garbage Collection Return Memory to the Operating System?

Can Garbage Collection Free Up Memory for the Operating System?

When a garbage collector (GC) runs, it cleans up unused memory within a process. A common misconception is that this memory is always kept within the process's memory pool for reuse, leading to the assumption that the memory of a process never truly decreases.

However, this is not the case for the HotSpot JVM used in Java. While it does release memory back to the operating system, it does so reluctantly due to the performance overhead of resizing the heap.

Factors Influencing Memory Release Behavior

The ability of the GC to shrink the heap depends on various factors:

• Garbage Collector: Different collectors have varying capabilities for shrinking the heap.
• JVM Version: Heap shrinking capabilities were introduced in later JVM versions.
• Collector Settings: Some additional options can be passed to enable heap shrinking for specific collectors.

Methods for Promoting Heap Shrinking

If heap shrinking is desired, several methods can be employed:

• Aggressive GC Configuration: Set JVM options like -XX:GCTimeRatio=19, -XX:MinHeapFreeRatio=20, and -XX:MaxHeapFreeRatio=30 to prompt GC runs and limit unused memory after each cycle.
• Concurrent GC: Set -XX:InitiatingHeapOccupancyPercent to a low value to trigger concurrent collections more frequently, which can consume CPU resources but accelerate heap shrinking.
• G1GC Modifications: In JDK 8, enable -XX:-ShrinkHeapInSteps to apply heap shrinking more aggressively.
• JDK 9: Enable prompt memory release for G1GC via G1PeriodicGCInterval.
• JDK 12: Similar prompt release behavior is enabled for ZGC by default.

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