Maximum Java heap size of a 32-bit JVM on a 64-bit OS

user_3916 0
Viewed 315636

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.

jvm java
17 Answers

The limitation also comes from the fact that for a 32 bit VM, the heap itself has to start at address zero if you want all those 4GB.

Think about it, if you want to reference something via:

0000....0001

i.e.: a reference that has this particular bits representation, it means you are trying to access the very first memory from the heap. For that to be possible, the heap has to start at address zero. But that never happens, it starts at some offset from zero:

    | ....               .... {heap_start .... heap_end} ... |
 --> (this can't be referenced) <--

Because heap never starts from address zero in an OS, there are quite a few bits that are never used from a 32 bits reference, and as such the heap that can be referenced is lower.

user_10593720
Related