Why am I able to set -Xmx to a value greater than physical and virtual memory on the machine on both Windows and Solaris?

Question

On a 64-bit Windows machine with 12GB of RAM and 33GB of Virtual Memory (per Task Manager), I\'m able to run Java (1.6.0_03-b05) with an impossible -Xmx setting of 3.5TB but

Answer 1

Just to re-enforce Pascal's answer--Be very careful in windows when specifying a high max memory size. I was working on a server project that required as much physical memory as possible, but once you are over physical ram, abysmal performance is not a good description of what happens--Hung machine might be better.

What happens (at least this is my evaluation of it after days of examining logs and re-running tests) is, Windows runs out of ram and asks all apps to free up what they can. When it asks Java, Java kicks off a GC. The GC touches all of memory (causing anything that has been swapped out to be swapped in). This in turn causes windows to run out of memory. Windows then sends a message to all apps asking them to free up what they can.... (recurse indefinitely)

This may not ACTUALLY be what is going on, but the fact that Java GC touches Very Old Memory at times makes it incompatible with paging.