A temporary memory (aka swap) is not strictly needed by a Linux system. However some Linux distributions refuse to install if not provided. In some cases changing or adding swap space on the fly (if a system has no swap space) is a seldom task to do 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 singe 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 oververview

free -m

Turn off the swap partition with the command which turns off all swap spaces

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 command smem -s swap can list swap used by processes. Sometimes when only root is logged in one can see a line like:

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

The 240 indicates that the process with PID 16940 uses swap for the user USER. Usually this means that the user is logged in. In some case this is not the case and the entry hangs around. When the user logs in again with ssh for example usually the ssh process will “inherit” the ‘swappiness’.

To understand which service is using this entry by systemd, one has to remember the PID and execute 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
0.1.1	2022-05-30	Change shell to bash, +history, +systemd –user
0.1.0	2022-05-27	Initial release