How to allow PSOldGen to expand to accommodate very large objects?

Question

I have a program that needs to allocate heap space for one very large object, but I find that I get an OutOfMemoryException when it seems that there is much more free heap than is required for a large object.

This test program demonstrates the problem:

 public class HeapTest { public static void main(String[] args) { final int size = Integer.parseInt(args[0]); System.out.println(size >> 20); final byte[] buf = new byte[size]; System.out.println(buf.length); } } 

If I run it as java -Xmx40M -XX:+PrintGCDetails HeapTest 28000000 , I get the following output:

 26 [GC [PSYoungGen: 317K->176K(9216K)] 317K->176K(36544K), 0.0007430 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] [GC [PSYoungGen: 176K->144K(9216K)] 176K->144K(36544K), 0.0004650 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [Full GC [PSYoungGen: 144K->0K(9216K)] [PSOldGen: 0K->115K(9216K)] 144K->115K(18432K) [PSPermGen: 2710K->2710K(21248K)], 0.0053960 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] [GC [PSYoungGen: 0K->0K(9216K)] 115K->115K(36544K), 0.0002010 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [Full GC [PSYoungGen: 0K->0K(9216K)] [PSOldGen: 115K->112K(15104K)] 115K->112K(24320K) [PSPermGen: 2710K->2708K(21248K)], 0.0078150 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at HeapTest.main(HeapTest.java:5) Heap PSYoungGen total 9216K, used 476K [0x00000000ff2b0000, 0x0000000100000000, 0x0000000100000000) eden space 7936K, 6% used [0x00000000ff2b0000,0x00000000ff327190,0x00000000ffa70000) from space 1280K, 0% used [0x00000000ffa70000,0x00000000ffa70000,0x00000000ffbb0000) to space 1280K, 0% used [0x00000000ffec0000,0x00000000ffec0000,0x0000000100000000) PSOldGen total 27328K, used 112K [0x00000000fd800000, 0x00000000ff2b0000, 0x00000000ff2b0000) object space 27328K, 0% used [0x00000000fd800000,0x00000000fd81c378,0x00000000ff2b0000) PSPermGen total 21248K, used 2815K [0x00000000f3000000, 0x00000000f44c0000, 0x00000000fd800000) object space 21248K, 13% used [0x00000000f3000000,0x00000000f32bff48,0x00000000f44c0000) 

The array I'm trying to create should be around 27344K, which is slightly larger than the size of the PSOldGen object space. However, the total unused space is about 54000K, which is almost double what is needed for my array. Since my (real) program runs on the array in place, it uses almost any memory beyond what is required for the array, therefore allocating twice as much memory as I need, and using only half of them seems wasteful.

Is there a way I can convince the JVM to let the old generation grow bigger than it seems by default?

Answer 1

You can use NewRatio , NewRatio , MaxNewSize and SurvivorRatio to adjust the size of Young Generation , indirectly also adjusting the size of the old generation .

In addition, if you are worried about Permanent Generation using too much memory, you can configure it using the MaxPermSize command line option.

For more information, see Configuring the garbage collection with the Java 5.0 [tm] virtual machine , especially the generations part.

For example, setting the Young Generation maximum size to 10M allows your test program to run successfully:

 java -Xmx40M -XX:+PrintGCDetails -XX:MaxNewSize=10M HeapTest 28000000 

displays the following result on my computer:

 26 28000000 Heap par new generation total 9216K, used 1499K [106810000, 107210000, 107210000) eden space 8192K, 18% used [106810000, 106986ed0, 107010000) from space 1024K, 0% used [107010000, 107010000, 107110000) to space 1024K, 0% used [107110000, 107110000, 107210000) concurrent mark-sweep generation total 30720K, used 27343K [107210000, 109010000, 109010000) concurrent-mark-sweep perm gen total 21248K, used 4501K [109010000, 10a4d0000, 10e410000)