java - Tools to view/solve Windows XP memory fragmentation

Question

We have a java program that requires a large amount of heap space - we start it with (among other command line arguments) the argument -Xmx1500m, which specifies a maximum heap space of 1500 MB. When starting this program on a Windows XP box that has been freshly rebooted, it will start and run without issues. But if the program has run several times, the computer has been up for a while, etc., when it tries to start I get this error:

Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.

I suspect that Windows itself is suffering from memory fragmentation, but I don't know how to confirm this suspicion. At the time that this happens, Task manager and sysinternals procexp report 2000MB free memory. I have looked at this question related to internal fragmentation

So the first question is, How do I confirm my suspicion? The second question is, if my suspicions are correct, does anyone know of any tools to solve this problem? I've looked around quite a bit, but I haven't found anything that helps, other than periodic reboots of the machine.

ps - changing operating systems is also not currently a viable option.

Answer 1

Unless you are running out of page file space, this issue isn't that the computer is running out of memory. The whole point of virtual memory is to allow the processes to use more virtual memory than is physically available.

Not knowing how the JVM handles the heap, it is a bit hard to say exactly what the problem is, but one of the common issues is that there isn't enough contiguous free address space available in your process to allow the heap to be extended. Why this would be a problem after the machine has been running a while is a bit confusing.

I've been working on a similar problem at work. I have found that running the program using WinDBG and using the "!address" and "!address -summary" commands have been invaluable in tracking down why a processes' virtual address space has become fragmented. You can also try running the program after reboot and using the "!address" command to take a picture of the address space and then do the same when the program no longer runs. This might clue you in on the problem. Maybe something simple as an extra DLL getting loading might cause the problem.