Is Swap Necessary?

johnnyb writes "Kernel Trap has a great conversation on swap, whether it's necessary, why swapless systems might seem faster, and an overall discussion of swap issues in modern computing. This is often an issue for system administrators, and this is a great set of posts about the issue."

Comments

swap rule!

[Coneasfast]

the rule is swap should be 1.5x your RAM! ;)

actually MS followed this rule, in win2k, the default swap size is set to exactly 1.5x your ram, was 176 for my 128mb system, and 384 for my 256mb system, not sure about XP though, someone fill me in
(yes, some great minds working at MS)

Re:swap rule!

[Admiral Burrito]

so turn down swappiness a bit to avoid Mozilla/Firefox from being paged out when you leave for 15 minutes.

I think there is a common misconception about how virtual memory works in modern OSs. That is that if the VM writes a page to swap space, you have to wait for it to read it off the disk before you can use it again. That is not necessarily so.

VMs may write pages to swap and still leave the page in RAM. The swapping is a pre-emptive measure, so that if it really does need to free that memory for other purposes later on it doesn't have to to swap it out to disk, because it is already done. Modern VMs will write pages to swap during idle periods for exactly this reason. If you really do have enough physical RAM, then the VM will never need to repurpose that page of RAM, and it can use the page from RAM without reading back from swap.

In your hypothetical example the VM may write pages from your browser's process to swap, but when you get back to your desk you shouldn't notice, because the pages will still be in RAM.

The only reason to turn down swappiness is if you are a victim of that misconception, or if you want to avoid disk activity during idle periods (like, on a laptop where the hard disk is set to spin down after inactivity).

Re:IMHO

[irokitt]

Linux has two properties that make swap a good thing (TM).

The first thing to remember is that, for many Linux users, they have a newer PC running Windows (or a Mac ;) and a less recent PC running a Linux distro. The RAM threshold is realistically around the 128-512 MB range. Those who are dual-booting on a brand new machine can use 1 GB, but the rest of us put up with less than that (I for one want to avoid MBR screwups and the hassle of communicating with NTFS, so I don't dual boot. I had a nasty GRUB incident, so I'm probably paranoid).

Finally, every Linux user that has compiled a kernel knows that it can really tax a system. Gentoo users also know how strenuous a XFree86 or KDE/Gnome compile can be. Being able to work on another terminal while compiling is one of the most beautiful things about *nix, and to do that on anything with less than 512 MB or 1 GB or RAM you want to have some swap.

And finally, while RAM is very cheap, so are hard drives, and how hard is it to squeeze a swap partition out of a hard drive? Can it really hurt that much to let the system use it?

As for Windows, swap is absolutely required for a lot of the games that are out there. I've heard that Unreal-engine based games in particular make heavy use of swap filing.

Re:I just don't get it.

[sprag]

The potential speed increase isn't seen when comparing 1G RAM vs a 2G RAM system. Its comparing a 1G RAM system with a 1G RAM system with swap.

The gist of it is: with swap you can put things that aren't being used (like mingetty, gdm, etc) into swap to free up space for things that are running now. Without swap you have to keep the little-used processes in memory and you don't have as much 'free' space to use for things like caches.

Its also important to note that the kernel will swap out code segments regardless of whether or not you have a swap partition: they get swapped out to nowhere. When they need to be swapped back in, the executable file itself is read.

The Kernel Can Take a Hint

[EventHorizon]

In the average case code and data _do_ tend to be accessed more than once. We would all be complaining a lot more if the kernel NEVER cached... remember the huge performance boost SMARTDRV made in DOS?

So, frankly, the default kernel behavior is right.

To fix the movie/updatedb/jumbo cp/etc issues see "man madvise" and check out MADV_DONTNEED. I am hoping applications will start using this syscall sooner, rather than later. The Linux VM can take a hint, and it's pretty easy to give it one.

"Swapiness"

[Compholio]

If you've got kernel 2.6 you can change the "swapiness" to fit your needs/desires. People with lots of RAM could experiment by changing the swapiness value to 0 and report back with the results (be easier than installing a system without swap).