I was playing recently with a recursive method for building all the possible routes via a set of nodes (with some predefined rules to link each pair of nodes) in Java. Rather than persisting the calculated routes into a file, I keep them in a HashSet.
Having about 138 nodes, this recursive method generates more than 5mln routes. This is quite an intensive calculation that consumes (and this is the point)
- about 1GB RAM with 32 bits JVM and
- about 2GB with 64 bits JVM.
Execution time, for building the routes, is like 17 seconds with 32 bits JVM and 21 seconds with 64 bits JVM.
Obvious question is "What the heck?". Apparently, here are the problems:
1. 64 bits systems are supposed to use (correct) 64 bits pointers (please allow me to use this C/C++ term) for accessing the process address space, thus allowing the process to use/access more (virtual) memory (i.e. 4GB with 32 bits vs. 16TB with 64 bits). But keeping an allocation table for 64 bits pointers (comparing to 32 bits) requires much more memory. So, 1GB vs. 2GB RAM isn't really a JVM problem.
Conclusion: if at some point you decide to use 64 bits JVM, don't forget to adjust the heap volume accordingly, in order to avoid unpleasant surprises.
2. Problem is also related to the way memory chips (DDR SDRAM) are made. Without diving into details (more details here http://lwn.net/Articles/250967/), accessing physical memory is a very slow operation and CPU tends to store frequently accessed data in its cache memory (L1, L2 and/or L3, which are far better memory chips, more expensive though). Modern compilers also tend to help developers from this point of view, if they follow some very well known rules (e.g. use arrays, stack is "cache-able", don't use "volatile" unless you really know what it is, etc.). But larger pointers require different memory alignment, thus 64 bits pointers aren't so cache efficient in the way 32 bits pointers are (e.g. when thinking about arrays). As a result, accessing physical RAM (or CPU cache miss) is the result of the 4 seconds difference.
To conclude: take care when deciding about using 32 bits or 64 bits Java.
And few online resources: