JVM Memory Management

#java

JVM Heap Structure

jvm heap

The JVM heap is divided into two parts: the Young Generation (nursery) and the Old Generation. The Young Generation is where new objects are allocated, consisting of three regions: Eden and two Survivor Spaces. Most new objects are placed in Eden, and when it fills up, a Minor GC moves surviving objects to one of the Survivor Spaces (S0, S1) or promotes them to the Old Generation after several cycles. The Old Generation holds long-lived objects and is cleaned up via Major GC, which is slower and occurs less frequently. Recent JVM versions introduced a keep area in the Young Generation to delay premature promotion of newly allocated objects, ensuring more efficient garbage collection.

Parameters:

  • -Xms: initial heap size. heap 시작
  • -Xmx: max heap size. heap 최대
  • -XX:MetaspaceSize: initial Metaspace size. Perm/Meta 기본
  • -XX:MaxMetaspaceSize: maximum Metaspace size. Perm/Meta 최대
  • Metaspace: Stores class metadata (replaced PermGen in Java 8).

Behavior

  • The JVM starts with the initial memory size and grows dynamically as needed, up to the maximum size.
  • If memory usage reaches the maximum and cannot grow further, the JVM may throw an OutOfMemoryError
  • example: -Xms512m -Xmx1024m:
    • The JVM starts with 512 MB of heap.
    • It can grow up to 1024 MB if needed

Tunning:

  • Tune -Xms & -Xmx close to reduce resizing overhead.
  • Adjust Metaspace for applications with heavy class loading (e.g., Spring/Hibernate).

Further Readings