I am new to the concept of virtual memory. I will like to know whats the maximum space of memory a procces can use through virtual memory? Is it available RAM + available swap space ??
(This question has already been asked but people had different opinions on this issue)
For most systems the maximum size of virtual memory is determined by the number of bits in a virtual address that are supported by the MMU.
For example; for typical 64-bit 80x86 CPUs a virtual address is 64 bits, but only the lowest 48 bits are supported by the MMU, so the virtual address size is 1 << 48 = 256 TiB. Everything else (amount of RAM, swap space, etc) doesn't matter.
In theory, you can (e.g.) fill the entire virtual address space by mapping the same page of RAM everywhere; and (for 64-bit 80x86) it would only cost 4 KiB of RAM (for the data that's mapped everywhere) plus another 16 KiB of RAM (for MMU's own data - page tables, etc). In other words, a measly 20 KiB of physical RAM is enough to fill a whopping 256 TiB of virtual space.
Of course for practical purposes often the kernel reserves some of the virtual address space (e.g. half of it), so a process can only use the remainder (e.g. 128 TiB); and (for modern 64-bit CPUs) unless you're mapping the same data at different places in the virtual address space (which would be silly/pointless) it's likely that you'll run out of things to put in a virtual address space before you run out of virtual address space.
This isn't the case for older 32-bit CPUs, where the virtual address space is a lot smaller (where maybe a process can only use 2 GiB out of a 4 GiB total space) and it's a lot easier to run out of space.
