Usage Report on Heap Memory Areas

Java GC Tutorials - Herong's Tutorial Examples

∟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:

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:

"def new generation total 3072K, used 319K ..." - "def new generation" is the internal name of the Young (New) generation managed by the Serial Collector. It is allocated with 3072K. And 319K is used at this moment.
"eden space 2752K, 0% used ..." - The "eden" space is allocated with 2752K with a usage of 0% at this moment.
"from space 320K, 99% used ..." - The "from" space is allocated with 128K with a usage of 99% at this moment.
"to space 320K, 0% used ..." - The "to" space is allocated with 320 KB with a usage of 0% at this moment.
"tenured generation total 6848K, used 739K ..." - "tenured generation" is the internal name of the Tenured (Old) generation managed by the Serial Collector. It is allocated with 6848K. And 739K is used at this moment.
"Metaspace used 2268K, ... reserved 1056768K" - The meta space (which is not part of the heap memory and not managed by the garbage collector) is allocated with 1GB.
"class space used 287K, ... reserved 1048576K" - The class space (which is not part of the heap memory and not managed by the garbage collector) is allocated with 1GB.

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.