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

Question

The question is not about the maximum heap size on a 32-bit OS, given that 32-bit OSes have a maximum addressable memory size of 4GB, and that the JVM's max heap size depends on how much contiguous free memory can be reserved.

I'm more interested in knowing the maximum (both theoretical and practically achievable) heap size for a 32-bit JVM running in a 64-bit OS. Basically, I'm looking at answers similar to the figures in a related question on SO.

As to why a 32-bit JVM is used instead of a 64-bit one, the reason is not technical but rather administrative/bureaucratic - it is probably too late to install a 64-bit JVM in the production environment.

Answer 1

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, try the above code and let us know.

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)