Java : Could not reserve enough space for object heap

Question

I am getting the following error when trying to run java with 1G memory.

C:\>java -verbose -Xmx1G

When I run it, I receive the following message:

Error occurred during initialization of VM
Could not reserve enough space for object heap
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

It is working fine with max memory set to 768M. I am using Java 7 (build 1.7.0_15-b03).

Here is my system's info

Total Physical Memory:     3,510 MB
Available Physical Memory: 2,393 MB
Virtual Memory: Max Size:  2,048 MB
Virtual Memory: Available: 1,965 MB
Virtual Memory: In Use:    83 MB
OS: Windows XP.

Answer 1

The heap does not only depend on the available RAM or physical memory. The maximum theoretical heap limit for the 32-bit JVM is 4G. Due to various additional constraints such as available swap, kernel address space usage, memory fragmentation, and VM overhead, in practice the limit can be much lower.