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Consider to following 1 line program:

public static void main(String[] args) throws Exception {
    // 134217728 * 2 bytes / 1024 / 1024 = 256M
    char[] array = new char[134217728]; 
}

How much memory does the JVM need to allocate this 256M character array?

Turns out the answer is -Xmx384m. Now lets try 512M character array...

// 268435456 * 2 bytes / 1024 / 1024 = 512M
char[] array = new char[268435456];

The answer appears to be -Xmx769m.

In running though a few examples for a character array of size m. The jvm needs at minimum 1.5m megabytes of memory to allocate the array. This seems like a lot, can anyone explain what is happening here?

user2140921
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    This depends on details of the implementation of the JVM, which will vary from JVM to JVM. – antlersoft Mar 13 '15 at 18:55
  • @antlersoft: While that's true, there are relatively few JVMs, and one that is rather more widely used than others, and fairly well documented... – Jon Skeet Mar 13 '15 at 18:59
  • Duplicate of http://stackoverflow.com/questions/24202523/java-process-memory-check-test – apangin Mar 13 '15 at 19:05

2 Answers2

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I believe you're observing the way that the Oracle JVM allocates memory.

In particular, the whole array has to fit into one "generation". By default, the old generation is twice the size of the new generation - which means for every 3MB you add in total, you only get 2MB of extra space in the old generation. If you change the ratio, you can allocate the char array in a smaller total size. For example, this works for me with your 512MB array:

java -Xmx530M -XX:NewRatio=50 Test

As an aside, you see exactly the same effect with a byte array, and then you don't need to worry about doubling the length of the array to get the size in bytes. (There's the small constant overhead for the class reference and the length, but obviously that's trivial.)

Jon Skeet
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The environment needs a little bit of space for itself and as in one comment it depends on the used JVM and compiler (OpenJdk or Oracle, java 6/7/8). All in all it should orient on the size: Character.SIZE * array.length.

neofu
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