How to check swap memory usage in Linux

Swap memory usage provides insight into how Linux handles pressure on physical RAM, especially on servers and desktops running many concurrent processes. When RAM fills up, inactive pages are moved to swap so that foreground workloads continue to run, but every page fault that hits disk introduces additional latency.

Under the hood, the kernel's virtual memory subsystem manages a set of swap devices and files, tracks which pages are resident, and exposes statistics through utilities such as free, swapon, and vmstat. These tools read data from interfaces like /proc/meminfo and /proc/swaps, presenting totals, per-device usage, and rates of swapping in and out.

Understanding swap usage requires more than a single number, because high swap consumption with little ongoing swap traffic is often benign, while continuous swapping usually points to memory pressure or misconfigured workloads. The commands below apply to most modern Linux distributions, and interpreting their output carefully helps decide whether tuning, adding RAM, or investigating memory-hungry processes is necessary.

1. Open a terminal emulator on the Linux system.
2. Run the free command to view overall memory and swap usage in human-readable units.

   $ free -h
                  total        used        free      shared  buff/cache   available
   Mem:            23Gi       1.2Gi        20Gi        13Mi       1.6Gi        21Gi
   Swap:          1.1Gi          0B       1.1Gi

   The Swap row shows total swap space, swap currently in use, and remaining free swap, which is useful for a quick high-level check.

3. List active swap devices and files with swapon.

   $ swapon --show
   NAME               TYPE  SIZE USED PRIO
   /var/lib/swap      file 1024M   0B   -2
   /mnt/data/swapfile file   64M   0B   -3

   The NAME column identifies each swap partition or file, while USED reveals how much swap is consumed on that specific device.

4. Display swap statistics directly from /proc/swaps for a kernel-level view.

   $ cat /proc/swaps
   Filename				Type		Size		Used		Priority
   /var/lib/swap                           file		1048572		0		-2
   /mnt/data/swapfile                      file		65532		0		-3

   The data in /proc/swaps backs commands like swapon and is suitable for scripts that parse raw fields.

5. Monitor swap activity over time with vmstat.

   $ vmstat 5 5
   procs -----------memory---------- ---swap-- -----io---- -system-- -------cpu-------
    r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st gu
    1  0      0 21727444 304124 1357144    0    0     4   256  509    0  0  0 99  0  0  0
    0  0      0 21722216 304124 1357148    0    0     0     1  640  466  2  1 97  0  0  0
    0  0      0 21726520 304132 1357148    0    0     0    15  974 1466  0  0 100  0  0  0
    0  0      0 21680968 304132 1357440    0    0     0  3091  332  201  1  0 99  0  0  0
    1  0      0 21680968 304140 1357440    0    0     0    10  726 1055  0  0 100  0  0  0

   The si and so columns show swap in and swap out rates in KiB per second, which should stay near zero on an idle or lightly loaded system.

6. Cross-check global swap totals using /proc/meminfo.

   $ grep -i swap /proc/meminfo
   SwapCached:            0 kB
   SwapTotal:       1114104 kB
   SwapFree:        1114104 kB
   Zswap:                 0 kB
   Zswapped:              0 kB

   SwapTotal and SwapFree should closely match the values reported by free, confirming that all swap devices are accounted for.

7. Use a real-time monitor such as htop to observe per-process memory and swap usage during load.

   $ htop

   Enable a SWAP column in htop via the Setup menu to identify which processes are actively using swap space.

8. Confirm swap usage and activity by ensuring the reported Swap values and si, so rates align with expectations for the current workload.