Minor GC Triggering Condition of Serial Collector

Java GC Tutorials - Herong's Tutorial Examples

∟Minor GC Triggering Condition of Serial Collector

This section describes the triggering condition of the Serial collector Minor GC process. The condition is the failure of new object allocation in 'eden'.

We know that when the "eden" space is full, the Serial collector will start a Young (Minor) GC. What is the exact definition of full? Is it 90% full or 100% full?

By looking at GC log messages, I think the exact definition is that "There is not enough free memory in "eden" to allocate the next new object." This is why the Serial collector uses "GC (Allocation Failure)" to label the Young GC event in the log message. "Allocation Failure" in "eden" triggers the Young GC.

If you turn on the "-XX:+PrintHeapAtGC" option, you may see different "eden" usage percentages before different Young GCs.

For example, the following log message shows a Young GC was triggered when the Serial collector failed to allocate the next new object. At that time, "eden" usage was 99%, which is about 10835K used out of 10944K. In other words, the next new object is larger than 109K (or 10944K-10835K).

[debug][gc,heap] GC(1) Heap before GC invocations=1 (full 0): def new ...
[debug][gc,heap] GC(1)   eden space 10944K,  99% used [...
[debug][gc,heap] GC(1)   from space 1344K, 100% used [...
[debug][gc,heap] GC(1)   to   space 1344K,   0% used [...
[debug][gc,heap] GC(1)  tenured generation   total 27328K, used 2025K ...
[debug][gc,heap] GC(1)    the space 27328K,   7% used [...
[info ][gc,heap] GC(1) DefNew: 12285K->1343K(12288K)
[info ][gc,heap] GC(1) Tenured: 2025K->7255K(27328K)
[info ][gc     ] GC(1) Pause Young (Allocation Failure)
                       13M->8M(38M) 9.055ms
...

Here is another example. The following log message shows a Young GC was triggered when the Serial collector failed to allocate the next new object. At that time, "eden" usage was 97%, which is about 39731K used out of 40960K. In other words, the next new object is larger than 1229K (or 40960K-39731K).

[debug][gc,heap] GC(2) Heap before GC invocations=2 (full 0): def new ...
[debug][gc,heap] GC(2)   eden space 40960K,  97% used [...
[debug][gc,heap] GC(2)   from space 40960K,   0% used [...
[debug][gc,heap] GC(2)   to   space 40960K,   0% used [...
[debug][gc,heap] GC(2)  tenured generation   total 122880K, used 39778K ...
[debug][gc,heap] GC(2)    the space 122880K,  32% used [...
[info ][gc,heap] GC(2) DefNew: 40129K->0K(81920K)
[info ][gc,heap] GC(2) Tenured: 39778K->62307K(122880K)
[info ][gc     ] GC(2) Pause Young (Allocation Failure)
                       78M->60M(200M) 11.046ms
...