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JMX, Scenarios, Business Value, ROI, Monitoring JMX, scenario, business value, ROI, use case 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
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.