Linux – Is it safe to share a swap partition with more than one Linux OS

Some of what you're describing sounds less like virtual memory and more like a disk cache, where the operating system keeps in-memory copies of the data it reads off of the disk, in case an application wants to read it again. In this scenario it may also delay writing "dirty" pages back to the disk in order to increase efficiency by writing a bunch of pages all at once.

Virtual memory is the other way around -- data is written into RAM first, and only into the swap file if the kernel needs to free up that RAM for some other purpose. (Not every bit of memory necessarily has a corresponding space in the swap file; otherwise you couldn't have a swap file smaller than your RAM size.) Conventional thinking is that this is the way it needs to be; it would be too expensive to keep copying the RAM to disk all the time when you don't need to. Think about how slow your computer runs when you're running lots of memory-hungry programs and it has to start thrashing the swap space.

That said, there are some experimental research operating systems that work more like you describe, with some complex rules for copying the memory back to disk at regular intervals, and the result is that there's no need for hibernate -- you can pull the plug on the computer and lose maybe 30 seconds of work. Check out CoyotOS, CapROS, or EROS if you're interested theoretically. I don't know what their performance is like, or what other downsides there might be to such a strategy, but when you factor in crazy things like memory shared with your video card or other devices, I'd imagine it's difficult to get it working properly for all cases. As far as I know, no production operating system does this sort of thing.

Unfortunately, no. The way Windows virtual memory system works, many of the pages simply can't be mapped until they fault and can't be stored in memory unless they're mapped. So Windows has to wait until the pages are accessed.