Question

I am running an application, using java 5 on Weblogic with the CMS Garbage Collector. In the garbage collection logs, I see logs of messages, most of which I can make sense of using the Memory Managements in Java HotSpot Virtual Machine document from Sun and this helpful blog entry (https://blogs.oracle.com/poonam/entry/understanding_cms_gc_logs).

However, there are some entries like this:

124525.178: [Full GC Before GC:
Statistics for BinaryTreeDictionary:
------------------------------------
Total Free Space: 353610889
Max   Chunk Size: 353610889
Number of Blocks: 1
Av.  Block  Size: 353610881
Tree      Height: 1
Statistics for IndexedFreeLists:
--------------------------------
Total Free Space: 4190038
Max   Chunk Size: 256
Number of Blocks: 37181
Av.  Block  Size: 119
 free=357800921 frag=0.0233
Before GC:
Statistics for BinaryTreeDictionary:
------------------------------------
Total Free Space: 0
Max   Chunk Size: 0
Number of Blocks: 0
Tree      Height: 0
Statistics for IndexedFreeLists:
--------------------------------
Total Free Space: 0
Max   Chunk Size: 0
Number of Blocks: 0
 free=0 frag=0.0000

I’ve run the file through HPjmeter and there don’t seem to be any problems, but the words ‘full gc’ worry me 🙂

What does “Full GC before GC” mean, and is it a stop-the-world event?

Answer 1

You are seeing two messages on the same line Full GC means its is starting a Full GC. Before GC means it is dumping the state of the system before it starts.