swappiness in Ubuntu: April 2010 Archives

First things first: My particular Ubuntu 10.04 LTS installation is not suffering from the Xorg memory leak.

I added mesa-utils so I could run:

glxinfo | grep "GLX version"

My output is:

GLX version: 1.2

If it was version 1.4, I'd have the newer, leaky Xorg, but the machine is properly reverted back to 1.2.

So what's my increased use of swap all about? I don't know if it's beneficial or not to have so much swapping going on, but a couple of readers have told me that Ubuntu's "swappiness" is set to a level of 60, which is optimal for servers. Desktops run better with lower "swappiness," and 10 is the suggested level.

All of this "swappiness" information is available in the Ubuntu community's Swap FAQ, which offers the following:

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What is swappiness and how do I change it?

The swappiness parameter controls the tendency of the kernel to move processes out of physical memory and onto the swap disk. Because disks are much slower than RAM, this can lead to slower response times for system and applications if processes are too aggressively moved out of memory.

* swappiness can have a value of between 0 and 100
* swappiness=0 tells the kernel to avoid swapping processes out of physical memory for as long as possible
* swappiness=100 tells the kernel to aggressively swap processes out of physical memory and move them to swap cache

The default setting in Ubuntu is swappiness=60. Reducing the default value of swappiness will probably improve overall performance for a typical Ubuntu desktop installation. A value of swappiness=10 is recommended, but feel free to experiment. Note: Ubuntu server installations have different performance requirements to desktop systems, and the default value of 60 is likely more suitable.

To check the swappiness value

cat /proc/sys/vm/swappiness

To change the swappiness value A temporary change (lost on reboot) with a swappiness value of 10 can be made with

sudo sysctl vm.swappiness=10

To make a change permanent, edit the configuration file with your favorite editor:

gksudo gedit /etc/sysctl.conf

Search for vm.swappiness and change its value as desired. If vm.swappiness does not exist, add it to the end of the file like so:

vm.swappiness=10

Save the file and reboot.

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At first I changed my swappiness temporarily. But now I'm ready make the fix permanent. Thanks for the tip, readers ctk and Mike. And thanks to the Ubuntu community for this nice little bit of how-to documentation on swap, plus the rest of the community documentation, all searchable, too.

A new way to sudo: I've never heard of gksudo before, but this FAQ recommend using it, and it worked perfectly. From the man page for gksudo:

gksu is a frontend to su and gksudo is a frontend to sudo. Their primary purpose is to run graphical commands that need root without the need to run an X terminal emulator and using su directly.

Works for me.

How other distros set their "swappiness": I've done a bit of checking, and it seems that most Linux distros, including Fedora, Debian, PCLinuxOS and Suse, also set their default swappiness at 60. Most of what I found about swappiness is in agreement with the Ubuntu FAQ, with desktop users setting swappiness to 10. I saw a few posts about MySQL servers that recommended setting swappiness to 0; I couldn't tell you anything about that.

What I will be doing is running with swappiness at 10. Whether or not I see or feel any change, I'll write again with an update.

Think about this: Sure, changing the swappiness from 60 to 10 is going to reduce what I saw as excessive swapping in Ubuntu. Will performance on the desktop really improve? And if swappiness has been at 60 in most distros for a long while, why am I suddenly seeing a problem in Ubuntu 10.04 that I never saw previously.

More to think about: The Linux kernel tends to take memory and hold onto it for what is presumably the greater good. As I understand it, the system isn't so much using all the memory you see it taking while monitoring a utility such at top. That memory will be used when the system needs it.

I don't begin to understand the gritty details of all this, nor of swap and swappiness, and I'm just comparing my experience now with that of running Linux distributions and BSD projects in the recent past.

Update, 3 p.m.: It's been a few hours since I made this change, and so far the system is using no swap and is running as well as or better than it did before.