Usage Report on Heap Memory Areas

This section provides explanations of the usage report on heap memory areas generated by the '-Xlog:gc+heap=debug' log option.

If we want to know the heap memory is divided into different areas, we can take a look at the heap usage report printed by the "-Xlog:gb+heap=debug" log option.

Rerun GarbageCollection.java with the "-Xlog:gb+heap=debug" option for a few seconds, and review the first "Heap after GC ..." messages:

C:\herong>java -Xms10m -Xmx10m -XX:+UseSerialGC -Xlog:gc+heap=info
   GarbageCollection
...
[debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
                        def new generation   total 3072K, used 319K [...
[debug][gc,heap] GC(0)   eden space 2752K,   0% used [...
[debug][gc,heap] GC(0)   from space 320K,  99% used [...
[debug][gc,heap] GC(0)   to   space 320K,   0% used [...
[debug][gc,heap] GC(0)  tenured generation   total 6848K, used 739K [...
[debug][gc,heap] GC(0)    the space 6848K,  10% used [...
[debug][gc,heap] GC(0)  Metaspace used 2268K,
                          capacity 6149K, committed 6400K, reserved 1056768K
[debug][gc,heap] GC(0)   class space used 287K,
						  capacity 672K, committed 768K, reserved 1048576K
...

Here is how we can read the heap usage report:

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.

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 - Tight Heap (Part 2)

 Serial GC Tracing - Tight Heap (Part 3)

 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" 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"

 The Z Garbage Collector (ZGC) - "+XX:+UseZGC"

 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

 References

 Full Version in PDF/EPUB