Java not using all free memory.

[abacathoo]

Hello,

So can someone explain why I see this behaviour?

Runtime runtime = Runtime.getRuntime();
System.out.println("free memory: " + runtime.freeMemory());
byte[] a = new byte[4000000];
System.out.println("free memory: " + runtime.freeMemory());

gives

free memory: 9854904
free memory: 5854888

(9854904-5854888 gives 4000016)

Runtime runtime = Runtime.getRuntime();
System.out.println("free memory: " + runtime.freeMemory());
int[] a = new int[1000000];
System.out.println("free memory: " + runtime.freeMemory());

gives

free memory: 9854904
free memory: 5854888

(9854904-5854888 gives 4000016 (exact same as before))

Runtime runtime = Runtime.getRuntime();
System.out.println("free memory: " + runtime.freeMemory());
byte[] a = new byte[4000000];
int[] b = new int[1000000];
System.out.println("free memory: " + runtime.freeMemory());

gives

free memory: 9854904
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at ***.***.***.***.***.***.mergetest.main(mergetest.java:35)

What on earth is going on?
Surely allocating those two arrays should only take up 8000032 bytes, well within the 9854904 java reports as having free...

[Eranga]

First of all, do you know how the memory works in a computer?

[abacathoo]

Surely the Java should hide anything about how memory works in a computer from me?

Am I being stupid here?

[JosAH]

Surely the Java should hide anything about how memory works in a computer from me?

Am I being stupid here?

This might be a silly question but is there something else (another Thread maybe?) that eats up the rest of the free memory?

kind regards,

Jos

[abacathoo]

I doubt it.

Runtime runtime = Runtime.getRuntime();
System.out.println("free memory: " + runtime.freeMemory());
int[] a = new int[1000000];
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());
System.out.println("free memory: " + runtime.freeMemory());

gives

free memory: 9854904
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888
free memory: 5854888

Which I also find surprising since I would thing the print statement itself would leave some rubbish on the heap that wouldn't get garbage collected yet.

[Tolls]

I've just run the second one (that fails with you), with -Xmx12M:

free memory: 8192808
free memory: 3141880

Which gives 5050928, which is a strange number, considering the individual ones are 4000024.

All I can say is, you're on the edge of your memory there so the JVM might be doing strange things. With larger memory than I gave there I got the expected 8000048 with the two arrays.

[abacathoo]

So is there a way of knowing exactly how much memory I can use without Java freaking out?
I could maybe hack something like.
long availiable_mem = r.freeMemory()-1000000;
I couldn't find any documentation anywhere about how much of the Java free memory you can run into.

[Tolls]

Don't worry about it.
You either need the memory or you don't, don't micro manage it in Java.

What are you doing such that you feel you need to do this?

[abacathoo]

An external sort, with the VM limited to 10M
It's an assignment, I think it's set in java just because the people who set it feel like having fun.
I want to load the biggest chunk from disk that I can without running into an OutOfMemoryException

[Tolls]

OK.
Don't push the boundary, because the JVM does more in the background than you can monitor. This sort of thing might make sense in C, but is pretty silly in Java, even on a mobile.

[abacathoo]

Thanks for your help.
Bye.