Understanding Linux Swap: How It Works, Tuning Tips, and Common Cleanup Commands

1. How Swap Works

In early computers memory was expensive and limited, so a disk area could be used as virtual memory called swap. Because disk I/O is much slower than RAM, reaching swap indicates memory pressure; high‑performance applications usually avoid swap by adding RAM.

Memory pages are classified as file pages (cached file data) and anonymous pages (heap, stack, etc.). Swap typically stores anonymous pages. Writing pages to swap is called “swap‑in” and reading them back “swap‑out”. If memory remains insufficient after reclamation, the OOM killer may terminate processes.

2. Memory Reclamation Mechanism

The kernel uses thresholds defined in /proc/sys/vm/min_free_kbytes and related values pages_low, pages_high, and pages_free. The daemon kswapd0 periodically scans memory and triggers reclamation based on where the free pages count falls relative to these thresholds.

If free memory < pages_min, only the kernel can allocate memory.

If free memory is between pages_min and pages_low, pressure is high; kswapd0 reclaims until free memory exceeds pages_high.

If free memory is between pages_low and pages_high, moderate pressure but new allocations can still be satisfied.

If free memory > pages_high, there is ample memory and no pressure.

3. Optimizing Swap Usage

Adjust /proc/sys/vm/min_free_kbytes to change the reclamation threshold.

Inspect /proc/zoneinfo to see memory distribution across zones (Normal, DMA, DMA32, etc.). Example output:

$ cat /proc/zoneinfo
...
Node 0, zone   Normal
 pages free   227894
 min          14896
 low          18620
 high         22344
...
 nr_free_pages          227894
 nr_zone_inactive_anon 11082
 nr_zone_active_anon   14024
 nr_zone_inactive_file 539024
 nr_zone_active_file   923986
...

Set /proc/sys/vm/swappiness to control the preference between swapping anonymous versus file pages. A value of 0 favors RAM, while 100 aggressively uses swap. The default on Linux is 60, meaning swap starts being used when memory usage exceeds about 40% of total RAM.

Even with swappiness set to 0, the kernel may still use swap under extreme pressure.

Temporary change:

sysctl -w vm.swappiness=1   # or 0

Permanent change (add to /etc/sysctl.conf and reload):

vm.swappiness = 0
sysctl -p

4. Quickly Identifying High‑Swap Processes

Sort processes by their VmSwap value to list the PID, name, and swap usage:

for file in /proc/*/status; do
  awk '/VmSwap|Name|^Pid/ {printf $2 " "}' $file
done | sort -k3 -n -r | head

This command outputs the processes that consume the most swap.

5. Common Swap Cleanup Methods

To clear swap, you can turn it off and on again:

swapoff -a && swapon -a

Note that forcing a swap reset while the system is under memory pressure may cause instability, so use with caution.