Question

The theoretical maximum heap value that can be set with -Xmx in a 32-bit system is of course 2^32 bytes, but typically (see: Understanding max JVM heap size – 32bit vs 64bit) one cannot use all 4GB.

For a 64-bit JVM running in a 64-bit OS on a 64-bit machine, is there any limit besides the theoretical limit of 2^64 bytes or 16 exabytes?

I know that for various reasons (mostly garbage collection), excessively large heaps might not be wise, but in light of reading about servers with terrabytes of RAM, I’m wondering what is possible.

Answer 1

If you want to use 32-bit references, your heap is limited to 32 GB.

However, if you are willing to use 64-bit references, the size is likely to be limited by your OS, just as it is with 32-bit JVM. e.g. on Windows 32-bit this is 1.2 to 1.5 GB.

Note: you will want your JVM heap to fit into main memory, ideally inside one NUMA region. That’s about 1 TB on the bigger machines. If your JVM spans NUMA regions the memory access and the GC in particular will take much longer. If your JVM heap start swapping it might take hours to GC, or even make your machine unusable as it thrashes the swap drive.

Note: You can access large direct memory and memory mapped sizes even if you use 32-bit references in your heap. i.e. use well above 32 GB.

Compressed oops in the Hotspot JVM

Compressed oops represent managed pointers (in many but not all places in the JVM) as 32-bit values which must be scaled by a factor of 8 and added to a 64-bit base address to find the object they refer to. This allows applications to address up to four billion objects (not bytes), or a heap size of up to about 32Gb. At the same time, data structure compactness is competitive with ILP32 mode.