Linux swap shenanigans

In this post we will cover a few linux swap recipes.

Empty swap space

Completely empty (flush) swap space:

% swapoff --all && swapon --all

Decrease swappiness

Emptying is too extreme. Why did you get so much swap in the first place? A small tweak is to decrease the sensibility of the system to swap:

$ cat /etc/sysctl.d/90-custom.conf
vm.swappiness=20
vm.vfs_cache_pressure=50

The default swappiness of the Linux kernel these days is 60%, which IMHO is quite aggressive for desktop usage. By decreasing it to 20%, our system will only start to swap once we use more than 80% of total RAM. In other words, only when there is 20% or less of free / available RAM.

vfs_cache_pressure:

This percentage value controls the tendency of the kernel to reclaim the memory which is used for caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to reclaim dentries and inodes at a “fair” rate with respect to pagecache and swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will never reclaim dentries and inodes due to memory pressure and this can easily lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 causes the kernel to prefer to reclaim dentries and inodes.

However, /etc/sysctl.d settings will only be applied after a reboot. To apply them immediately, use the sysctl(8) command:

% sudo sysctl -p /etc/sysctl.d/90-custom.conf
vm.swappiness = 20
vm.vfs_cache_pressure = 50

Use a swapfile

If you find yourself with a fully partitioned disk without any dedicated swap partition, there’s a trick to adding swap anyway: Use a swap file! Everything is a file anyway!

# https://wiki.archlinux.org/title/Swap#Swap_file

# Create the swap file: 8GiB in this case, to match our total RAM
% dd if=/dev/zero of=/swapfile bs=1M count=8000 status=progress

# Set restricting permissions
% chmod 600 /swapfile

# Format the ~~partition~~ file
% mkswap /swapfile

# Activate the swap file
% swapon /swapfile

You can check it’s working correctly by inspecting /proc/swaps:

% cat /proc/swaps
Filename				Type		Size		Used		Priority
/swapfile                               file		8388604		0		-2

Then finally add it to your /etc/fstab so that it is automatically mounted in subsequent boots:

# swap file
/swapfile none swap defaults 0 0

Add ZRAM swap

Explaining zram is out of scope if this post, but check out the ArchWiki or Wikipedia.

The recipe I use in Arch Linux is the zramswap package:

1. Install the package.
2. Set desired zram swap percentage, I picked 20%:

% cat /etc/zramswap.conf
ZRAM_SIZE_PERCENT=20

3. Enable/Start the service:

% systemctl enable --now zramswap
% systemctl status zramswap
● zramswap.service - Zram-based swap (compressed RAM block devices)
     Loaded: loaded (/usr/lib/systemd/system/zramswap.service; enabled; vendor preset: disabled)
     Active: active (exited) since Tue 2022-02-01 16:13:37 EST; 7h ago
   Main PID: 582 (code=exited, status=0/SUCCESS)
        CPU: 27ms

Feb 01 16:13:37 localhost.localdomain systemd[1]: Starting Zram-based swap (compressed RAM block devices)...
Feb 01 16:13:37 localhost.localdomain zramctrl[627]: Setting up swapspace version 1, size = 1.5 GiB (1654009856 bytes)
Feb 01 16:13:37 localhost.localdomain zramctrl[627]: LABEL=zram0, UUID=a39e0131-f102-4503-a1e7-a3e0ca330126
Feb 01 16:13:37 localhost.localdomain systemd[1]: Finished Zram-based swap (compressed RAM block devices).

You can inspect /proc/swaps again to check it’s working properly¹:

% cat /proc/swaps
Filename				Type		Size		Used		Priority
/swapfile                               file		8388604		0		-2
/dev/zram0                              partition	1615244		0		100