Virtually contiguous vs. physically contiguous memory

Question

Is virtually contiguous memory also always physically contiguous? If not, how is virtually continuous memory allocated and memory-mapped over physically non-contiguous RAM blocks? A detailed answer is appreciated.

Answer 1

Short answer: You need not care (unless you're a kernel/driver developer). It is all the same to you.

Longer answer: On the contrary, virtually contiguous memory is usually not physically contiguous (only in very small amounts). Except by coincidence, or shortly after the machine has just booted. That isn't necessary, however.

The only way of allocating larger amounts of physically contiguous RAM is by using large pages (since the memory within one page needs to be contiguous). It is however a useless endeavor, since there is no observable difference for your process whether memory of which you think that it is contiguous is actually contiguous, but there are disadvantages to using large pages.

Memory mapping over phyically non-contiuous RAM works in no particularly "special" way. It follows the same method which all memory management follows.

The OS divides virtual memory in "pages" and creates page table entries for your process. When you access a memory in some location, either the corresponding page does not exist at all, or it exists and corresponds to a real page in RAM, or it exists but doesn't correspond to a real page in RAM.

If the page exists in RAM, nothing happens at all¹. Otherwise a fault is generated and some operating system code is run. If it turns out the page doesn't exist at all (or does not have the correct access rights), your process is killed with a segmentation fault.

Otherwise, the OS chooses an arbitrary page that isn't used (or it swaps out the one it thinks is the least important one), and loads the data from disk into that page. In the case of a memory mapping, the data comes from the mapped file, otherwise it comes from swap (and for completely new allocated memory, the zero page is copied). The OS then returns control back to your process. You never know this happened.

If you access another location in a "contiguous" (or so you think!) memory area which lies in a different page, the exact same procedure runs.

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¹ In reality, it is a little more complicated, since a page may exist in RAM but not exist "officially", being part of a list of pages that are to be recycled or such. But this gets too complicated.

Answer 2

No, it doesn't have to. Any page of the virtual memory can be mapped to an arbitrary physical page. Therefore you can have adjacent pages of your virtual memory pointing to non-adjacent physical pages. This mapping is maintained by the OS and is used by the MMU unit of CPU.