What is Swap memory and How to clear Swap usage in Linux ?

What is Swap Space ?

Swap is a space on disk that is used by the system when the available memory in the RAM (physical memory) is completely utilized. This is basically to increase the available virtual memory in the system. The swap memory will be used once the physical memory is full. Since this is residing in the disk, the processing speed using this memory will be slow compared to the processing in physical memory (RAM).

Why we need swap space ?

Suppose we have a system with 4GB RAM. When we start the system the memory usage will be less. But as we open applications or start running processes, the memory utilization will increase. If it reaches the 4GB utilization, we will not be able to use any additional applications and we will have to wait to get some free space in the RAM. With swap memory, the allocated space in the disk will be used in case of any additional requirement and the applications will still run even after crossing the max limit of system RAM. As already explained, the performance of swap will be very slow as compared to RAM.

How the memory management works internally ?

The Linux kernel has a memory management process. This process monitors all the processes and identifies the less frequently used memory pages (or blocks). In case of additional memory requirement exceeding the RAM limit comes, this memory management program will utilize the space in system hard disk allocated for “swapping” or paging these less frequently used memory blocks. In this way the RAM will be freed up and the active memory for running live application will become available in the system.

How to clear the swap memory usage?

If you want to clear the swap memory, you can execute the following command in the terminal as root user.

swapoff -a && swapon -a

WARNING.!!!: Be careful doing this, as this may affect your system’s stability, especially if its already low on RAM. Better not to set these swap clearing scripts as cronjob.


Linux commands to check the disk utilization, size of directory or file

  • Command to check the disk utilization
df -h

The ‘-h’ option will provide the utilization in human readable format.

  • Command to check the size of a directory
du -sh <directory name>
  • Command to check the size of a file
du -sh <file name>
  • Command to check the size of files in a directory

Go inside the directory and execute the following command

du -sh *