Is virtual memory so precious? about brk()

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http://www.unix.com – We know for every process, it has 4G virtual memory on a 32bit machine. since virtual memory is not physical memory, why don't operating system allocate all it's virtual memory to it, but set a "program break" to limit it's heap space? Even if the operating system allocate all the 4G virtual memory to a process, only when the process access an address that not mapped into physical memory, the OS allocate physical memory to it. So, what is the reason to limit a process's virtual memory space by "program break"? Thanks. (HowTos)