Why Swapoff is So Darned Slow

Most people who’ve installed Linux are familiar with the idea of a swap partition. That odd little slice of the hard disk that you can’t use for a drive. The kernel’s virtual memory system uses it as an overflow tank for RAM (it gets used for other things, too, such as when you put your computer into hibernation). Most personal computers only have one, but there can actually be up to 32 of them.

Occasionally, a system administrator might need to take one offline. That’s what the swapoff command does. At any given time, an active swap partition will be holding any number of memory pages; and before it’s deactivated, all those pages will need to be swapped back into RAM, and the page tables of the programs that are using them will have to be updated so that the programs can find them. It sounds like it should be easy, given enough free RAM to hold them all, but many of them are being used by more than one program. Think of the program that draws your window buttons on the screen, for instance. Instead of writing that program into RAM over and over and taking up a lot of room, all the programs that need a window can use the same copy of it.

The way it works now, swapoff looks at each swapped out memory page in the swap partition, and tries to find all the programs that use it. If it can’t find them right away, it will look at the page tables of every program that’s running to find them. In the worst case, it will check all the page tables for every swapped out page in the partition. That’s right–the same page tables get checked over and over again. This is a huge waste of effort. One user said it took over four hours for swapoff to run.

Yes, there is a better way. That’s my OPW project. Stay tuned.

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