Release Java Heap Sapce [duplicate]

Question

I have an issue with java performance. I tried using GC1 but it didn't make things great. My only question is why the JVM keeps the java heap space even if it's not used ? As you can see in the picture first the batch work finish around 10:30 but the garbage collector wasn't triggered to clean. So I did it manually at 10:52 then the used space was freed but heap (8GB) is still allocated by the JVM. Why ?

Answer 1

I've seen this trend too while doing the exact same thing that you did (and see here). The explanation is quite simple: JVM will not give back memory to the OS, even if it has lots of it free. I don't know the exact details of why this is so, but it has to do with the fact that asking memory back from the OS is expensive (this is ultra-simplified).

So I guess that the simple answer here is that - this is normal.

Here it is! I found where I have actually read about this.

Answer 2

G1GC perform whole-heap operations, such as global marking, concurrently with the application threads. This prevents interruptions proportional to heap.

Through G1GC is a new age Garbage collection algorithm, a switch from CMS or Old age garbage collection should be only made in case of following :

• More than 50% of the Java heap is occupied with live data.
• The rate of object allocation rate or promotion varies significantly.
• Undesired long garbage collection or compaction pauses (longer than 0.5 to 1 second)

More details about G1 internal working can be found at : http://docs.oracle.com/javase/7/docs/technotes/guides/vm/G1.html