Is Virtual memory really useful all the time?

Question

Virtual memory is a good concept currently used by modern operating systems. But I was stuck answering a question and was not sure enough about it. Here is the question:

Suppose there are only a few applications running on a machine, such that the physical memory of system is more than the memory required by all the applications. To support virtual memory, the OS needs to do a lot work. So if the running applications all fit in the physical memory, is virtual memory really needed?

(Furthermore, the applications running together will always fit in RAM.)

Answer 1

Even when the memory usage of all applications fits in physical memory, virtual memory is still useful. VM can provide these features:

• Privileged memory isolation (every app can't touch the kernel or memory-mapped hardware devices)
• Interprocess memory isolation (one app can't see another app's memory)
• Static memory addresses (e.g. every app has main() at address 0x0800 0000)
• Lazy memory (e.g. pages in the stack are allocated and set to zero when first accessed)
• Redirected memory (e.g. memory-mapped files)
• Shared program code (if more than one instance of a program or library is running, its code only needs to be stored in memory once)

Answer 2

While not strictly needed in this scenario, virtual memory is about more than just providing "more" memory than is physically available (swapping). For example, it helps avoiding memory fragmentation (from an application point of view) and depending on how dynamic/shared libraries are implemented, it can help to avoid relocation (relocation is when the dynamic linker needs to adapt pointers in a library or executable that was just loaded).

Answer 3

A few more points to consider:

• Buggy apps that don't handle failures in the memory allocation code
• Buggy apps that leak allocated memory

Virtual memory reduces severity of these bugs.