Maximum Java heap size of a 32-bit JVM on a 64-bit OS

Question

You can ask the Java Runtime:

public class MaxMemory {
    public static void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        long totalMem = rt.totalMemory();
        long maxMem = rt.maxMemory();
        long freeMem = rt.freeMemory();
        double megs = 1048576.0;

        System.out.println ("Total Memory: " + totalMem + " (" + (totalMem/megs) + " MiB)");
        System.out.println ("Max Memory:   " + maxMem + " (" + (maxMem/megs) + " MiB)");
        System.out.println ("Free Memory:  " + freeMem + " (" + (freeMem/megs) + " MiB)");
    }
}

This will report the “Max Memory” based upon default heap allocation. So you still would need to play with -Xmx (on HotSpot). I found that running on Windows 7 Enterprise 64-bit, my 32-bit HotSpot JVM can allocate up to 1577MiB:

[C:scratch]> java -Xmx1600M MaxMemory
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
[C:scratch]> java -Xmx1590M MaxMemory
Total Memory: 2031616 (1.9375 MiB)
Max Memory:   1654456320 (1577.8125 MiB)
Free Memory:  1840872 (1.75559234619 MiB)
[C:scratch]>

Whereas with a 64-bit JVM on the same OS, of course it’s much higher (about 3TiB)

[C:scratch]> java -Xmx3560G MaxMemory
Error occurred during initialization of VM
Could not reserve enough space for object heap
[C:scratch]> java -Xmx3550G MaxMemory
Total Memory: 94240768 (89.875 MiB)
Max Memory:   3388252028928 (3184151.84297 MiB)
Free Memory:  93747752 (89.4048233032 MiB)
[C:scratch]>

As others have already mentioned, it depends on the OS.

For 32-bit Windows: it’ll be <2GB (Windows internals book says 2GB for user processes)
For 32-bit BSD / Linux: <3GB (from the Devil Book)
For 32-bit MacOS X: <4GB (from Mac OS X internals book)
Not sure about 32-bit Solaris, but the code above has been tested in this answer.

For a 64-bit host OS, if the JVM is 32-bit, it’ll still depend, most likely like above as demonstrated.

— UPDATE 20110905: I just wanted to point out some other observations / details:

The hardware that I ran this on was 64-bit with 6GB of actual RAM installed. The operating system was Windows 7 Enterprise, 64-bit
The actual amount of Runtime.MaxMemory that can be allocated also depends on the operating system’s working set. I once ran this while I also had VirtualBox running and found I could not successfully start the HotSpot JVM with -Xmx1590M and had to go smaller. This also implies that you may get more than 1590M depending upon your working set size at the time (though I still maintain it’ll be under 2GiB for 32-bit because of Windows’ design)

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