How Swap Works

Swap gets a bad reputation — "my system is swapping, it's slow." But swap is a nuanced tool. Understanding when Linux uses it, why it makes certain choices, and how to tune it can turn a crashing server into a stable one.

What is Swap?

What is swap space? Swap is disk space that Linux can use as overflow memory. When physical RAM fills up, the kernel moves less-recently-used memory pages to the swap partition or swap file. This frees RAM for active processes. Later, if that swapped-out page is needed again, the kernel reads it back from disk.

# Check swap usage
free -h
#              total    used    free    shared  buff/cache  available
# Mem:           32G     12G      8G      500M        12G       19G
# Swap:           8G    512M      7G

# See swap details
cat /proc/swaps
# /dev/sda2  partition  8388604  524288  -2

# Create a swap file (if you don't have a partition)
sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Which Pages Get Swapped — LRU

The kernel doesn't randomly choose pages to swap. It uses an LRU (Least Recently Used) algorithm: pages that haven't been accessed recently are candidates for eviction. Linux maintains two lists per memory zone:

Active list: Recently accessed pages — stay in RAM
Inactive list: Not recently accessed — candidates for swap or reclaim

Pages move from active → inactive when not accessed for a while. Inactive file-backed pages (from /tmp, logs) are dropped (re-read from disk if needed). Inactive anonymous pages (heap, stack) go to swap.

Swappiness — Tuning the Aggressiveness

What is vm.swappiness? A kernel tunable (0-200, default 60) that controls how aggressively Linux swaps anonymous pages vs evicting file cache. Lower value = prefer to keep pages in RAM, only swap when necessary. Higher value = swap more aggressively, keep file cache.

# Check current swappiness
cat /proc/sys/vm/swappiness  # typically 60

# Desktop: reduce swappiness (prioritize responsiveness)
sudo sysctl vm.swappiness=10

# Server with plenty of RAM: reduce further
sudo sysctl vm.swappiness=1

# Persistent:
echo "vm.swappiness=10" | sudo tee -a /etc/sysctl.conf

zswap and zram — Compressed Memory

Modern alternatives that avoid slow disk I/O:

Feature	zswap	zram
What it is	Compressed swap cache in RAM	Compressed block device in RAM as swap
How it helps	Compresses pages before writing to disk swap. Fewer disk writes.	Swap to RAM (compressed) instead of disk. Much faster.
Typical compression	2-3x	2-4x
Best for	Systems with slow disk swap	Systems with no swap, low RAM (RPi, laptops)

# Enable zram (creates a compressed RAM swap device)
sudo modprobe zram
echo lz4 > /sys/block/zram0/comp_algorithm
echo 4G > /sys/block/zram0/disksize
sudo mkswap /dev/zram0
sudo swapon -p 100 /dev/zram0  # -p 100 = higher priority than disk swap