What does swappiness do?

• Swappiness is a property for the Linux kernel that changes the balance between swapping out runtime memory, as opposed to dropping pages from the system page cache. Swappiness can be set to values between 0 and 100, inclusive. A low value means the kernel will try to avoid swapping as much as possible where a higher value instead will make the kernel aggressively try to use swap space.

• As this parameter sets the kernel's balance between reclaiming pages from the page cache and reclaiming pages by swapping out process memory, a discussion of page reclaim overall is warranted. The reclaim code works (in a very simplified way) by calculating a few numbers:

• The distress value is a measure of how much trouble the kernel is having freeing memory. The first time the kernel decides it needs to start reclaiming pages, distress will be zero; if more attempts are required, that value goes up, approaching a max value of 100.

• The mapped_ratio value is an approximate percentage of how much of the system's total memory is mapped (i.e. is part of a process's address space) within a given memory zone.

• And vm_swappiness is the swappiness parameter, set to 60 by default.

• With these numbers in hand, the kernel calculates its "swap tendency":

swap_tendency = mapped_ratio/2 + distress + vm_swappiness;

• If swap_tendency is below 100, the kernel will only reclaim page cache pages. Once it goes above that value, however, pages which are part of some process's address space will also be considered for reclaim. So, if the system's distress value is low and swappiness is set to 60, the system will not swap process memory until 80% of the total RAM in the system is allocated. Users who would like to never see application memory swapped out can set swappiness to a low value, say 5 or 10, causing the kernel to ignore process memory until the distress value gets quite high.

• Overall, increasing this value will make the system more inclined to utilize swap space, leaving more memory free for caches. Decreasing this value will make the system less inclined to swap, and may improve application responsiveness.

• Tuning vm.swappiness incorrectly may hurt performance, or may have a different impact between light and heavy workloads. Changes to this parameter should be made in small increments, and should be tested under the same conditions that the system normally operates.

• How to change it?

The swappiness parameter value is stored in a simple configuration text file located in /proc/sys/vm and is named “swappiness”. If you navigate there through the file manager, you will be able to locate the file and open it to check your system's swappiness. You can also check it or change it through the terminal (which is faster) by typing the following command: “sudo sysctl vm.swappiness=10” or whatever else between “0” and “100” instead of the value “10” that I used. To ensure that the swappiness value was correctly changed to the desired one, you simply type: “cat /proc/sys/vm/swappiness” on the terminal again and the active value will be outputted.

This change has an immediate effect in your system's operation and thus no rebooting is required. In fact, rebooting will revert the swappiness back to its default value (60). If you have thoroughly tested your desired swapping value and you found that it works reliably, you can make the change permanent by navigating to /etc/sysctl.conf which is yet another text configuration file. You may open this as root (administrator) and add the following line on the bottom to determine the swappiness: vm.swappiness=”your desire value here”. Then, save the text file and you're done!

• Factors for consideration

There are some maths involved in the swappiness that should be considered when changing your settings. The parameter value set to “60” means that your kernel will swap when RAM reaches 40% capacity. Setting it to “100” means that your kernel will try to swap everything. Setting it to 10 (like I did on this tutorial) means that swap will be used when RAM is 90% full, so if you have enough RAM memory, this could be a safe option that would easily improve the performance of your system.

Some users though want the full cake and that means that they set swapping to “1” or even “0”. “1” is the minimum possible “active swapping” setting while “0” means disable swapping completely and only revert to when RAM is completely filled. While these settings can still theoretically work, testing it in low-spec systems of 2GB RAM or less may cause freezes and make the OS completely unresponsive. Generally, finding out what the golden means between overall system performance and response latency requires quite some experimentation (as always).