Default NewRatio - Old vs. New Generation

Java GC Tutorials - Herong's Tutorial Examples

∟Default NewRatio - Old vs. New Generation

This section describes the default NewRatio which is the ratio of old generation size vs. new generation size.

From the last tutorial, we learned that the Serial GC will automatically divide the heap memory into 2 generations: 30% for the "new" (or Young) generation and 70% for the "old" (or Tenured) generation. In this tutorial, let's run more tests to confirm this:

1. Heap size: 10 MB -

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 [...
...

2. Heap size: 20 MB -

herong> java -Xms20m -Xmx20m -XX:+UseSerialGC -Xlog:gc+heap=info \
   GarbageCollection
...
[gc,heap] GC(0) Heap after GC invocations=1 (full 0):
                 def new generation   total 6144K, used 639K [...
[gc,heap] GC(0)   eden space 5504K,   0% used [...
[gc,heap] GC(0)   from space 640K,  99% used [...
[gc,heap] GC(0)   to   space 640K,   0% used [...
[gc,heap] GC(0)  tenured generation   total 13696K, used 1444K [...
[gc,heap] GC(0)    the space 13696K,  10% used [...
...

3. Heap size: 40 MB -

herong> java -Xms40m -Xmx40m -XX:+UseSerialGC -Xlog:gc+heap=info \
   GarbageCollection
...
[debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
                        def new generation   total 12288K, used 1343K [...
[debug][gc,heap] GC(0)   eden space 10944K,   0% used [...
[debug][gc,heap] GC(0)   from space 1344K,  99% used [...
[debug][gc,heap] GC(0)   to   space 1344K,   0% used [...
[debug][gc,heap] GC(0)  tenured generation   total 27328K, used 2024K [...
[debug][gc,heap] GC(0)    the space 27328K,   7% used [...
...

4. Heap size: 80 MB -

herong> java -Xms80m -Xmx80m -XX:+UseSerialGC -Xlog:gc+heap=info \
   GarbageCollection
...
[debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
                        def new generation   total 24576K, used 2688K [...
[debug][gc,heap] GC(0)   eden space 21888K,   0% used [...
[debug][gc,heap] GC(0)   from space 2688K, 100% used [...
[debug][gc,heap] GC(0)   to   space 2688K,   0% used [...
[debug][gc,heap] GC(0)  tenured generation   total 54656K, used 3308K [...
[debug][gc,heap] GC(0)    the space 54656K,   6% used [...
...

The table below is the summary of above tests:

Heap Size   New Generation   Tenured Generation
---------   --------------   ------------------
      10M        3072K 30%            6848K 70%
      20M        6144K 30%           13696K 70%
      40M       12288K 30%           27328K 70%
      80M       24576K 30%           54656K 70%

Conclusion: By default, Serial GC gives 30% of heap to the "new" generation, and 70% to the tenured generation. Or the default radio of "old" generation size and "new" generation size is 2.33. This ratio is also called NewRatio.