JVM Heapsize on 32 bit OS

I am using 开发者_开发知识库32 bit win.7 and using Eclipse. Also having 4GB RAM.

I want to allocate my java application a maximum heapsize of around 3 GB, but I am able to allocate maximum 1.5GB through VM arguments -Xmx1056m.

What should I do? If I Install a 64 bit win.7. it would be able then to allocate 3GB heapsize to my app?

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A regular 32-bit Windows process can only address 2GB of memory, even if you have more memory available. You can find the memory limits for different Windows versions here.

Since the VM need memory for more things than just the heap, the max heap size will be slightly less than the maxmimum memory available to the process. Usually, you can tweak the heap up to around 1.6GB for a 32-bit Windows VM.

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You need a 64bit OS and 64bit VM to allocate this much RAM.

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I don't have the link right now that describes the JVM memory management process. The limitation you have stumbled upon is a limitation of how Java performs garbage collection. The Java memory heap must be a contigious block. The garbage collection algorithms were optimized for this design limitation so they can perform efficiently. In a 32bit operating system, you are not in control of what memory addresses device drivers are loaded into. On Windows, the OS will only reallocate the device driver base address stored in the DLL if it conflicts with an already loaded code. Other operating systems may reallocate all device drivers on load so they live in a contiguous block near the kernel.

Basically, the limitation is the same for 64bit and 32bit operating systems. It's just on a 64bit OS there are several more address ranges to choose from. Make sure you use a 64bit JVM to match the OS. That's why the 64bit OS is able to find a larger contigious block of memory addresses than the 32bit OS.

EDIT: Additionally the 32bit JVM has a max heap size limit of 2GB.

REFERENCE:

http://publib.boulder.ibm.com/infocenter/javasdk/tools/index.jsp?topic=/com.ibm.java.doc.igaa/_1vg00014884d287-11c3fb28dae-7ff6_1001.html

Java maximum memory on Windows XP

http://forums.sun.com/thread.jspa?messageID=2715152#2715152

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What you will need is not only a 64bit OS and a 64bit VM, but also more memory.

On a 32bits Windows system the virtual address space is split with 2 GB for kernel operations and 2 GB for user applications. So you're screwed.

There's one possible but very unlikely workaround: you can enable the /3GB switch to raise this limitation and have the system allocate 1GB of virtual address space for for kernel operations and 3GB for user applications (if they are /LARGEADDRESSPACEAWARE).

Unfortunately, the 32bits Sun/Oracle HotSpot JVM isn't LARGEADDRESSAWARE (that I know of), and other 32bits JVM likely aren't either.

But think about it: even if you were able to do that, you would use all the memory available for you system. Nothing would be left for other programs after you've allocated your 3GB of heap for your JVM. Your system would be swapping to disk all the time. It would be unusable.

Just get a 64bis OS with more RAM. That's all there is for you, short of finding ways to have your program use less memory.