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JMX, FAQ, Troubleshooting, Common Questions JMX, FAQ, troubleshooting, common questions 3. What's the difference between monitoring heap "used" vs "committed"?

3. What's the difference between monitoring heap "used" vs "committed"?

3. What's the difference between monitoring heap "used" vs "committed"?

Three key heap metrics:

  • Heap Used: Actual memory consumed by Java objects currently in heap (live objects + garbage awaiting collection). Example: 3.4 GB used = application allocated 3.4 GB for objects, arrays, data structures. Alert on this metric for OutOfMemoryError prevention (Used >85% of Max = Warning).

  • Heap Committed: Memory reserved by JVM from OS (guaranteed available for heap allocation). Always >= Used, <= Max. Example: Used 3.4 GB, Committed 4 GB = JVM reserved 4 GB from OS, currently using 3.4 GB. JVM grows committed automatically as heap fills (starts at Min, grows toward Max).

  • Heap Max: Maximum heap size configured via -Xmx JVM argument. Example: -Xmx4g = Max 4 GB. Cannot exceed this value—OutOfMemoryError thrown if Used reaches Max.

Why monitor both:

  • Used >85% of Max: Proactive alert before heap exhaustion
  • Committed growing rapidly: Indicates JVM allocating more memory (potential memory leak, investigate if Committed approaching Max)
  • Used close to Committed: Heap fully utilized within reserved space (JVM will attempt to grow Committed, or trigger GC if at Max)

Example scenario: -Xms1g -Xmx4g (Min 1 GB, Max 4 GB). Application starts: Committed 1 GB, Used 0.5 GB. After 2 hours: Committed 2.5 GB (JVM auto-grew), Used 2.3 GB (normal growth). After 6 hours: Committed 3.8 GB, Used 3.7 GB (approaching Max). Alert fires "Heap Used 92% of Max" (3.7 GB / 4 GB) → Investigate memory leak or increase Max to 8 GB.

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