A simple “Hello World” needs 10G virtual memory on a 64-bit machine vs 1G at 32-bit?

Question

Running a simple Java program on our production machine, I noticed that this program eats up more 10G virt. I know that virtual memory is not that relevant, but at least I would

Answer 1

It turns out that on a modern computer architecture that uses virtual memory addressing (where the "memory space" an application sees does not actually relate to memory that's actually physically allocated), it really doesn't matter how much of this virtual "memory space" is given to an application upon startup. It doesn't mean that this much memory has been allocated by the system.

If an application sees a virtual address space 10GB large all it signals to the app is that it may use memory addresses up to 10GB if it wants. However, memory is not actually allocated in physical RAM until it is actually written to, and this is done on a page-by-page basis, where a page is a 4kB section of memory. The virtual address space, is just that - completely virtual until actually used.

Let's say an application is given 10GB of address space and it starts using some of it. As a "fresh" - previously untouched - page of this virtual memory is first written to, the system will, on a low level, "map" this virtual page to a section of physical memory, then write it. But that application itself does not have to worry about such details, it just acts as if it has full access to a virtual area of memory.

In the case of Java applications, it's not the application itself but Java that is allocated that address space, and Java simply requests a huge address space by default - the amount it requests is calculated relative to the physical memory size, but not because it has any need to be conservative, but just for practicality - an application is probably not going to want enough heap size to totally bring a server to its knees, so it's operating on the assumption it won't. As I said above this does not mean that this much is "allocated" or that the system has had to expend many resources doing so.