Comment by maushu | Hacker News Reader

Comment by maushu | Hacker News Reader

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I'm a developer too and this is what I understand (might be wrong):

- Virtual memory is basically "abstract memory" that is linked (mapped) to RAM or HDD, this means that by accessing this "memory" you might actually be accessing the HDD and, because of this, larger than physical ram.

- Virtual set size is the allocated virtual memory (the above) to the process.

- Resident set size is the allocated physical (RAM) memory to the process.

- Shared memory is memory that is shared on multiple processes, meaning that if you have 10 processes using 10mb of resident memory each and have 2mb of shared memory each, the total of resident memory used is not 100mb but 82mb.

Virtual memory is actually more subtle than that.

At a mechanical level, virtual memory is permission from the operating system to use addresses in your address space. It is so called because, as you point out, it allows us to separate the concept of "memory for a process" from "physical memory on a chip." The reason I further refine the concept is that allocating virtual memory does not allocate actual memory. Let's look at an example:

  void* addr = mmap(NULL, 10 * 1024 * 1024, PROT_READ | PROT_WRITE, 
                    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

That call allocates 10 MB of virtual memory. But there is no physical memory backing any of it. All that has happened is that the operating system has now said "Okay, starting at the address I return to you, you can now access 10 MB of memory. I will do all of the work of making sure physical memory backs the virtual memory when you access it." That is, once I try to access the memory, it will trigger a page fault, and the OS will find a page in physical memory to back my virtual memory. But until that happens, no memory - not in RAM, not on disk - backs the virtual memory.

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