Usage Report on Heap Memory Areas

This section provides explanations of the usage report on heap memory areas generated by the '-XX:+PrintGCDetails' option.

If the "-XX:+PrintGCDetails" option is used, HotSpot JVM also generates a nice summary report on usages of different Heap memory areas when the application is terminated.

Rerun GarbageCollection.java with the "-XX:+PrintGCDetails" option for a few seconds, and press Ctrl-C to terminate it. You will see the usage report of heap memory areas on the console:

C:\>\progra~1\java\jdk1.8.0\bin\java -Xms10m -Xmx10m -XX:+UseSerialGC
   -XX:+PrintGCDetails -XX:+PrintGCTimeStamps GarbageCollection

Step/TotalMemory/FreeMemory/UsedMemory:
1   10158080   8629864   1528216
2   10158080   7563632   2594448
2.148: [GC (Allocation Failure) 2.150: [DefNew: 2533K->320K(3072K), ...
3   10158080   6685688   3472392
4   10158080   5593720   4564360
4.164: [GC (Allocation Failure) 4.165: [DefNew: 2964K->320K(3072K), ...
5   10158080   4554136   5603944
6   10158080   3460808   6697272
(Ctrl-C)

Heap
 def new generation   total 3072K, used 2053K [0x04a00000, ...
  eden space 2752K,  62% used [...
  from space 320K, 100% used [...
  to   space 320K,   0% used [...
  
 tenured generation   total 6848K, used 4596K [0x04d50000, ...
   the space 6848K,  67% used [0x04d50000, 0x051cd3b8, ...

 Metaspace  used 1561K, capacity 2242K, committed 2368K, reserved 4480K

Here is how we can read the heap area usage report in the "heap" section:

Note that when the heap size is specified as 10 MB by -Xms10m -Xmx10m, the JVM is automatically gave 3072K to the "new" generation, and 6848K to the tenured generation. It's about 30% for the "new" generation, and 70% for the tenured generation.

Last update: 2018.

Table of Contents

 About This Book

 Heap Memory Area and Size Control

 JVM Garbage Collection Logging

 Introduction of Garbage Collectors

Serial Collector - "+XX:+UseSerialGC"

 What Is Serial Collector

 GC Log Message Format for Serial Collector

 GC Log Message Examples of Serial Collector

 Log Message Types from Serial Collector

 Serial Collector Stops Application for Minor/Major GC

Usage Report on Heap Memory Areas

 Default NewRatio - Old vs. New Generation

 "-XX:NewRatio" - Ratio of Tenured and "new" Generation

 "-XX:SurvivorRatio" - Ratio of Eden and Survivor Space

 Serial GC Tracing - Tight Heap

 Serial GC Tracing - Plenty of Heap

 Serial GC Tracing - Aged Live Objects

 Serial GC Tracing - Tenuring Threshold

 "-XX:TargetSurvivorRatio" - Second Tenuring Condition

 Serial GC Tracing - Tenuring Threshold Controlled

 "-XX:+NeverTenure" and "-XX:+AlwaysTenure" not Working

 Minor GC Triggering Condition of Serial Collector

 Parallel Collector - "+XX:+UseParallelGC"

 Concurrent Mark-Sweep (CMS) Collector - "+XX:+UseConcMarkSweepGC"

 Garbage First (G1) Collector - "+XX:+UseG1GC"

 Object References and Garbage Collection

 Garbage Collection Performance Test Program

 Performance Tests on Serial Collector

 Performance Tests on Parallel collector

 Performance Tests on Concurrent collector

 Performance Tests on G1 collector

 Garbage Collection Performance Test Summary

 Outdated Tutorials

 References

 Full Version in PDF/EPUB