Variable maximum heap sizes on Windows ?!?!?

Question

We run under 1.4.2 and often have to bump the memory settings up for large computations. We've found that depending on the machine, the maximum value for the -Xmx setting can vary significantly. On some machines, we can go as high at 1.4GB, on others only up to 850MB. The 850MB maximum was on a machine with 2GB of RAM running XPsp2.
What factors could be fragmenting the memory space of the VM such that it can't get a large contiguous allocation?

Answer 1

In a previous article there was a good link to memory limitations of windows. See http://www.unixville.com/~moazam/ I did a search on the internet using google http://www.google.com/search?q=memory+dll+view and the first link google provides is a free process viewer you use to see where dll are loaded in memory for a particular exe. http://www.xmlsp.com/pview/prcview.htm I downloaded and tried that program. You can right click on the java.exe and select Modules or Memory and it will give you list of all the loaded dll's and where they are located in memory. You can then see if it is a dll that is loaded in the middle of memory that is breaking up the contiguous memory. You may also want to run it on the different boxes to see if the dll are loaded differently on the individual boxes.
There are probably even better process viewers that could graphically show you the memory the modules are taken up, but I will allow you to do the further research.