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The MMU maps pages of your address space onto physical frames, so allocating memory usually just reserves address range. Touching a page for the first time causes a page fault the kernel resolves by finding a frame — which is why a large allocation can be instant and the first write to it slow. It also explains mmap, copy-on-write after fork, and why RSS is smaller than virtual size.
A process uses virtual addresses. The operating system and hardware map regions of that address space to physical memory or backing storage as needed, with page tables mediating access. Reserving an address range is not the same as filling every page with committed physical memory. Many systems map pages lazily, so the first access can trigger a page fault that the kernel resolves. But allocation behavior varies by allocator, operating system, and mapping flags; do not promise that every large allocation is instant.
This model explains several useful observations.
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