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

The limitations of a 32-bit JVM on a 64-bit OS will be exactly the same as the limitations of a 32-bit JVM on a 32-bit OS. After all, the 32-bit JVM will be running In a 32-bit virtual machine (in the virtualization sense) so it won't know that it's running on a 64-bit OS/machine.

The one advantage to running a 32-bit JVM on a 64-bit OS versus a 32-bit OS is that you can have more physical memory, and therefore will encounter swapping/paging less frequently. This advantage is only really fully realized when you have multiple processes, however.

Answer 2

You don't specify which OS.

Under Windows (for my application - a long running risk management application) we observed that we could go no further than 1280MB on Windows 32bit. I doubt that running a 32bit JVM under 64bit would make any difference.

We ported the app to Linux and we are running a 32bit JVM on 64bit hardware and have had a 2.2GB VM running pretty easily.

The biggest problem you may have is GC depending on what you are using memory for.

Answer 3

32-bit JVMs which expect to have a single large chunk of memory cannot use more than 4 Gb (since that is the 32 bit limit which must be for pointers). This includes Sun and - I'm pretty sure - also IBM implementations. I do not know if e.g. JRockit or others have a large memory option with their 32-bit implementations.

If you expect to be hitting this limit you should strongly consider starting a parallel track validating a 64-bit JVM for your production environment so you have that ready for when the 32-bit environment breaks down. Otherwise you will have to do that work under pressure, which is never nice.

Answer 4

Should be a lot better

For a 32-bit JVM running on a 64-bit host, I imagine what's left over for the heap will be whatever unfragmented virtual space is available after the JVM, it's own DLL's, and any OS 32-bit compatibility stuff has been loaded. As a wild guess I would think 3GB should be possible, but how much better that is depends on how well you are doing in 32-bit-host-land.

Also, even if you could make a giant 3GB heap, you might not want to, as this will cause GC pauses to become potentially troublesome. Some people just run more JVM's to use the extra memory rather than one giant one. I imagine they are tuning the JVM's right now to work better with giant heaps.

It's a little hard to know exactly how much better you can do. I guess your 32-bit situation can be easily determined by experiment. It's certainly hard to predict abstractly, as a lot of things factor into it, particularly because the virtual space available on 32-bit hosts is rather constrained.. The heap does need to exist in contiguous virtual memory, so fragmentation of the address space for dll's and internal use of the address space by the OS kernel will determine the range of possible allocations.

The OS will be using some of the address space for mapping HW devices and it's own dynamic allocations. While this memory is not mapped into the java process address space, the OS kernel can't access it and your address space at the same time, so it will limit the size of any program's virtual space.

Loading DLL's depends on the implementation and the release of the JVM. Loading the OS kernel depends on a huge number of things, the release, the HW, how many things it has mapped so far since the last reboot, who knows...

In summary

I bet you get 1-2 GB in 32-bit-land, and about 3 in 64-bit, so an overall improvement of about 2x.

Answer 5

I have tried setting the heap size upto 2200M on 32bit Linux machine and JVM worked fine. The JVM didnt start when I set it to 2300M.

Answer 6

From http://java.sun.com/performance/reference/whitepapers/tuning.html#section4.1.2 : "For a 32-bit process model, the maximum virtual address size of the process is typically 4 GB, though some operating systems limit this to 2 GB or 3 GB. The maximum heap size is typically -Xmx3800m (1600m) for 2 GB limits), though the actual limitation is application dependent. For 64-bit process models, the maximum is essentially unlimited. "

Found a pretty good answer here: http://stackoverflow.com/questions/171205/java-maximum-memory-on-windows-xp

Answer 7

On Solaris the limit has been about 3.5 GB since Solaris 2.5. (about 10 years ago)