Swappiness

Swappiness is a configuration value for the Linux kernel that controls how your kernel will behave when it comes to swap memory. This setting is often referred to as vm.swappiness. You can obtain your current swappiness value by reading from the /proc/sys/vm/swappiness file:

$ cat /proc/sys/vm/swappiness
60

The setting falls in the range \(swappiness \in \mathbb{Z} \cap [0, 100]\), i.e. in the (inclusive) range of 0 to 100.

How the setting affects swap

Your memory on your Linux machine is divided into different mappings. (Read more about Page mapping). Your anonymous and file-backed memory both can be cached to an effect on your disk and in your RAM, respectively.

Inside the Linux kernels source code, you will find the following:

/*
* With swappiness at 100, anonymous and file have the same priority.
* This scanning priority is essentially the inverse of IO cost.
*/
anon_prio = swappiness;
file_prio = 200 - anon_prio;

Which means that

  1. \(anon\_prio \in \mathbb{Z} \cap [0, 100]\)
  2. \(file\_prio \in \mathbb{Z} \cap [100, 200]\)
  3. As \(anon\_prio\) increases, \(file\_prio\) decreases.

Setting vm.swappiness=0 does not disable swap, nor does setting swappiness=100 max out swap usage. What it does however, is control how the prioritization of file-backed memory vs anonymous memory, and thereby how much swap to focus on storing on the disk or storing in memory.

To be clear:

  • The lower the value, the more willing the kernel is to give up anonymous memory pages and store it on disk in favor of file-backed memory.

  • The higher the value, the more willing the kernel is to give up file-backed memory pages and need to retrieve files from disk more often in favor of not needing to store as much anonymous memory on disk.

  • You cannot tell the kernel to prefer file-backed pages over anonymous pages. If you set vm.swappiness=100, then both anon_prio=100 and file_prio=100 so they receive equal amount of prioritization.

Changing your computers swappiness

The sysctl is of great help here. You can change the current swappiness directly without needing to reboot your computer by running:

$ sudo sysctl vm.swappiness=45
vm.swappiness = 45

The above will take action immediately.

The file /etc/sysctl.conf or file inside the /etc/sysctl.d/ directory usually has your computers swappiness written down. If you were to refresh these config files or reboot your machine, the swappiness setting will be reset to factory default or whatever is written in those files.

To make the swappiness change persitent, you need to save it in such a file. This requires super-user (sudo) access. For example:

# Check if swappiness is already set in one of the mentioned files
$ grep --recursive swappiness /etc/sysctl.conf /etc/sysctl.d
# no output? => not specified in any .conf file.

# This assumes you don't have swappiness written down
$ echo "vm.swappiness=60" | sudo tee /etc/sysctl.d/20_swappiness.conf

References

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