Java process memory check test

Question

I tried to see how -Xmx and -Xms parameters impact on my program and check how much memory does my process consume.

I wrote a simple program but am not able to reason out the results. Kindly help.

public static void main( String[] args ) {
        char[] array = new char[69926904];
}

I ran with parameters -Xms5M -Xmx200M. Ideally, since a character takes 2 bytes, it should hold 100M characters before exceeding the memory limit. Even if we say, few space is getting used for pointer and length, I don't know, why it is throwing error after 69926904 length.

Thanks.

Answer 1

Read carefully JVM Garbage Collection Tuning Guide about Generational Heap, and it will hopefully answer your question.

Run Java with -XX:+PrintGCDetails option, and everything should become clear:

Heap
 PSYoungGen      total 3584K, used 294K [0x00000000fbd60000, 0x00000000fc160000, 0x0000000100000000)
  eden space 3072K, 9% used [0x00000000fbd60000,0x00000000fbda9860,0x00000000fc060000)
  from space 512K, 0% used [0x00000000fc0e0000,0x00000000fc0e0000,0x00000000fc160000)
  to   space 512K, 0% used [0x00000000fc060000,0x00000000fc060000,0x00000000fc0e0000)
 PSOldGen        total 136576K, used 136576K [0x00000000f3800000, 0x00000000fbd60000, 0x00000000fbd60000)
  object space 136576K, 100% used [0x00000000f3800000,0x00000000fbd60000,0x00000000fbd60000)
 PSPermGen       total 21248K, used 2595K [0x00000000ee600000, 0x00000000efac0000, 0x00000000f3800000)
  object space 21248K, 12% used [0x00000000ee600000,0x00000000ee888db0,0x00000000efac0000)

Your 200M Java Heap consists of 2 generations: 1/3 (66.7M) is YoungGen and 2/3 (133.3M) is OldGen.

-XX:NewRatio option allows to change the proportion, but the default value of 2 means that YoungGen will reserve 1/(2+1) part of the heap.

Java objects cannot span generations, so the maximum size of object cannot be larger than the largest generation. In your case the largest generation is OldGen: 136576K = 139853824 which is exactly the size of char[69926904] (16 bytes header + 2 * 69926904 bytes of data).