Swap

Christian Külker

0.1.2

2023-05-25

Temporary memory (also known as swap) is not really needed by a Linux system. However, some Linux distributions refuse to install if it is not provided. In some cases, changing or adding swap space on the fly (when a system has no swap space) is an infrequent task, and sometimes the correct commands need to be remembered or looked up (in this document).

Linux provides two types of swap space. By default, most Linux installations use or create a swap partition, but it is also possible to use a specially configured file as a swap file.

Swap Games With Files

See how full the disk is

df -h

Prints the summary (if any) of swap spaces

swapon -s

See how many main memory and swap is available

free
free -m
cat /proc/swaps
top

See the swap in and out

vmstat

See the swap usage of single applications

smem -s swap

In case there is no swap and we want to create 2G of swap space

fallocate -l 2G /swapfile

See the result

ls -lh /swapfile

Swap space should not be readable by world

chmod 600 /swapfile

Create a swap signature

mkswap /swapfile

Use the swap file

swapon /swapfile

Show a summary

swapon -s

This should be visible in the memory overview

free -m

Disable the swap partition with the command that disables all swap space

swapoff -a

For some non Debian systems: To mount the swap file on boot, add an entry to /etc/fstab

echo "/swapfile none swap sw 0 0" >>/etc/fstab

For Debian systems: To mount the swap file on boot, add an entry to /etc/fstab

echo "/swapfile swap swap defaults 0 0" >>/etc/fstab

Swap Games with systemd

The smem -s swap command can list the swap space used by processes. Sometimes, if only root is logged in, you may see a line like this:

16940 USER     /lib/systemd/systemd --user      240     1376     2444     8088

The 240 indicates that the process with PID 16940 is using swap for the user USER. Usually this means the user is logged in. In some cases this is not the case and the entry is hanging. For example, if the user logs in again using ssh, the ssh process will usually “inherit” the swappiness.

To understand which service is using this entry from systemd, remember the PID and run the following command:

systemd-cgls -u user.slice --no-pager

It will print something like:

Unit user.slice (/user.slice):
├─user-0.slice
│ ├─session-34535.scope
│ │ ├─  619 /bin/login -p --
│ │ └─28392 systemd-cgls -u user.slice --no-pager
│ └─user@0.service
│   └─init.scope
│     └─16940 /lib/systemd/systemd --user
└─user-1000.slice
  └─user@1000.service
    └─init.scope
      └─11826 /lib/systemd/systemd --user

After executing a restart and systemd-cgls

systemctl restart systemd-user-sessions.service
systemd-cgls -u user.slice --no-pager

The entry gets shorter

Unit user.slice (/user.slice):
└─user-0.slice
  ├─session-34535.scope
  │ ├─  619 /bin/login -p --
  │ └─31767 systemd-cgls -u user.slice --no-pager
  └─user@0.service
    └─init.scope
      └─16940 /lib/systemd/systemd --user

And smem -s swap will not show the line for the user USER any more.

History

Version Date Notes
0.1.2 2023-05-25 Improve writing, spelling
0.1.1 2022-05-30 Change shell to bash, +history, +systemd –user
0.1.0 2022-05-27 Initial release
  • Swap