Warning: You should not use swapon on a swapfile with holes. If there is a swapfile with holes can be found in /var/log/systemlog. Look for:

swapon: swapfile has holes.

cat /proc/swaps
swapon -s

Commands:

cat /proc/meminfo 
free
top
htop

You can make your system a bit faster by clearing the swapspace
Do, it can take a bit time before the swap is cleared, as root

swapoff /dev/sdax
swapon /dev/sdax

x is the number of the partition / drive
or set swap usage to 0 kilobytes by doing as root

swapoff --all && swapon --all

See also swapoff --help for details about the --all option (differs from the man page)
See also man swappon for details about the --all option

When you, temporarily, need more swap space you can add an extra swapfile (to the already existing swap partition)
The util-linux package has to be installed. It should be installed when Debian is installed. This package contains, among others, fallocate, mkswap, swapon and swapoff
As root do

• fallocate -l 5G /mnt/5GExtraSWAP
• mkswap /mnt/5GExtraSWAP . This results in something like
  • mkswap: /mnt/5GExtraSWAP: insecure permissions 0644, 0600 suggested.
  • Setting up swapspace version 1, size = 5 GiB (524283904000 bytes)
  • no label, UUID=00000000-0000-0000-0000-000000000000
• chmod 600 /mnt/5GExtraSWAP
• swapon /mnt/5GExtraSWAP . This turns the swap on
• When you want to make it permanent then add to /etc/fstab : /mnt/5GExtraSWAP none swap sw defaults,pri=100 0 0 . pri stands for priority and can range from 0 to 32676. Give each swap partion or file the appropriate priority

swap off

swapoff -av
swapoff /mnt/5GExtraSWAP
swapoff /dev/sda4

swapon -av

swapon: /dev/sda4: found signature [pagesize=4096, signature=swap]
swapon: /dev/sda4: pagesize=4096, swapsize=2097152000, devsize=2097152000
swapon /dev/sda4

/mnt/5GExtraSWAP is not turned on. You need to do swapon -av /mnt/5GExtraSWAP. Then you get something like

swapon: /dev/sda4: found signature [pagesize=4096, signature=swap]
swapon: /dev/sda4: pagesize=4096, swapsize=2097152000, devsize=2097152000
swapon /dev/sda4
swapon: /mnt/5GExtraSWAP: found signature [pagesize=4096, signature=swap]
swapon: /mnt/5GExtraSWAP: pagesize=4096, swapsize=5474836480, devsize=5474836480
swapon /mnt/5GExtraSWAP

From

sgdisk --info=3 /dev/sda

Partition GUID code: 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F (Linux swap)
or the
Partition unique GUID: FD3B975F-8477-4C5A-AF57-E1D63FAB7845
tried in fstab

PARTUUID=FD3B975F-8477-4C5A-AF57-E1D63FAB7845  none    swap  sw      0 0                       
PARTUUID=0657FD6D-A4AB-43C4-84E5-0933C84B4F4F  none    swap  sw      0 0

Running as root

swapon -a

results in

swapon: cannot find the device for PARTUUID=0657FD6D-A4DD-43A4-74E5-0933C84B4F4F

should work according to the fstab man page. It does not.

In fstab: PARTLABEL=swap

PARTLABEL=swap  none  swap  sw  0 0

Running as root

swapon -a

results in

swapon: /dev/sda3: read swap header failed

altough this should work according to the man page of fstab. It does not.

Running

swapoff

results in

swapoff failed: Cannot allocate memory

Solution: An application is still using swap space. Stop the application and try again

On systems with a SSD, SDcard, CFcard or similar enough memory it is needed to avoid swapping
The swappiness can then better be set low or off
Do vi /etc/sysctl.conf and add or change

vm.swappiness = x

where x is a value in the range 0 or 100
0 means the swap is off
The higher x the more the computer might swap
To activate execute

sysctl -p (Load the settings from /etc/sysctl.conf)

To check the swap intensity

 sysctl vm.swappiness

Swap
Swappiness on Wikipedia
Swappiness
Reload sysctl.conf on linux using sysctl

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