What is virtual memory, and what does it give the operating system?

With virtual memory, every process sees its own private address space. The CPU has a memory management unit (MMU). It translates virtual addresses to physical memory using page tables, one set per process. Memory is split into fixed-size pages, usually 4 KB on x86-64.

What this gives the operating system:

• Isolation. A process can only reach pages mapped in its own tables. So it can't read another process's memory.
• Lazy allocation. A page gets physical RAM only when the process first touches it. That first touch triggers a page fault, and the kernel then gives it RAM.
• Sharing. Processes can share pages, like the code of a shared library.
• Copy-on-write. After fork(), parent and child share pages. A page is copied only when one of them writes to it.
• Swap. When RAM runs low, the kernel can drop page cache or move anonymous pages (memory not backed by a file, like the heap) to swap on disk. A later page fault brings them back.

This is why a process's virtual size (VSZ) can be far larger than its resident size (RSS, the part actually in RAM). It is also why heavy swapping makes a machine very slow.