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On a Debian Linux 3.16 machine, I have 244 MB of swap space used:

# free -h total used free shared buffers cached Mem: 94G 36G 57G 1.9G 3.8G 11G -/+ buffers/cache: 20G 73G Swap: 487M 244M 243M

Looking at this, I cannot find 244 MB used. 

# for file in /proc/*/status ; do grep VmSwap $file; done | sort -nk 2 | tail VmSwap: 0 kB VmSwap: 0 kB VmSwap: 0 kB VmSwap: 0 kB VmSwap: 0 kB VmSwap: 0 kB VmSwap: 4 kB VmSwap: 12 kB VmSwap: 16 kB VmSwap: 36 kB

And I only have 34 MB of SwapCached: 

# grep -i swap /proc/meminfo SwapCached: 34584 kB SwapTotal: 499708 kB SwapFree: 249388 kB

Kernel doc says about this:

SwapCached: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O)

How can I know which process is using my swap space on my Linux system? More precisely: Where are consumed each of those 244 MB of swap?

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  • SwapCached can also be seen as stuff that was swapped out, and was later needed. And you don't have much swapped out, and can not have much swapped out. Why do you have a swap space at all? Commented Jul 23, 2018 at 12:24
  • @ctrl-alt-delor Without any swap space, I've seen many processes just crash "randomly". So I think that's a nice safety even if you have a lot of RAM. Commented Oct 9, 2020 at 17:02

3 Answers 3

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How can I know which process is using my swap space on my Linux system?

Swap space is not necessarily used by specific processes.

More precisely: Where are consumed each of those 244 MB of swap?

Files stored on tmpfs based file systems might be using them (tmpfs first uses RAM as back-end but, not to waste RAM, can paginate out to the swap area blocks that are not actively used).

Check the output of :

df -ht tmpfs
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    This is the answer which applies for me! Is there a way to know how much of this tmpfs storage is currently in the swap space? Maybe it can be inferred from /proc/meminfo. (?) Commented Aug 8, 2018 at 14:46
  • I don't understand how the output of df -ht tmpfs could help with determining that it uses swap space. Also in my case, most just use a mere 1% of the available space which I would imagine is not going to be swapped out? Commented Oct 9, 2020 at 17:00
  • @AlexisWilke tmpfs stores its content in virtual memory. Virtual memory is a combination of RAM and disk, more precisely of page cache and swap area. If there are files in such file systems and if these files are seldom used, regardless of whether they use 1% or more of the available space, the OS might rightly decide to move their content from the page cache to the swap area. There is no point to waste RAM with unused data. Commented Oct 9, 2020 at 20:41
  • That doesn't answer my question, though. The df command is not going to tell me whether the tmpfs module decided to swap some data to disk. I would think there isn't really a way to know. Commented Oct 10, 2020 at 6:53
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    @AlexisWilke There are definitely cases where you can be sure some of the tmpfs used data is stored on swap area pages. There is also an indirect method, more or less convenient depending on the machine activity. Just read the files stored in tmpfs. If that triggers disk activity, some or all content was on disk. Of course, this is a "destructive" test because at the end, the data will tend to stay on RAM, at least momentarily. Commented Oct 10, 2020 at 13:47
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The /proc/PID/smaps is an extension based on maps, showing the memory consumption for each of the process's mappings. For each of mappings there is a series of lines such as the following:

08048000-080bc000 r-xp 00000000 03:02 13130 /bin/bash Size: 1084 kB Rss: 892 kB Pss: 374 kB Shared_Clean: 892 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 0 kB Referenced: 892 kB Anonymous: 0 kB LazyFree: 0 kB AnonHugePages: 0 kB ShmemPmdMapped: 0 kB Shared_Hugetlb: 0 kB Private_Hugetlb: 0 kB Swap: 0 kB SwapPss: 0 kB KernelPageSize: 4 kB MMUPageSize: 4 kB Locked: 0 kB VmFlags: rd ex mr mw me dw

Try

SWAP_FIELD="SwapPss" #SWAP_FIELD="Swap" for proc in /proc/*; do if [[ ! "${proc}" =~ /proc/[0-9]+/* ]]; then continue fi executable=$(readlink "${proc}/exe" | awk '{print $1}') awk -v executable="${executable}" \ -v SWAP_FIELD="${SWAP_FIELD}" \ '$0~SWAP_FIELD{swap+=$2}END{print swap"\tKiB\t"executable}' < "${proc}/smaps"; done |\ sort -n |\ awk '{total+=$1}/[0-9]/;END{print total "\tKB\tTotal"}'

Example output:

0 KB /usr/bin/bash 0 KB /usr/bin/bash 0 KB /usr/bin/bash 0 KB /usr/bin/bash 0 KB /usr/bin/bash 0 KB /usr/bin/docker-containerd 0 KB /usr/bin/docker-containerd-shim 0 KB /usr/bin/docker-containerd-shim 0 KB /usr/bin/docker-proxy 0 KB /usr/bin/docker-proxy 0 KB /usr/bin/docker-proxy 0 KB /usr/bin/gawk 0 KB /usr/bin/readlink 0 KB /usr/bin/sleep 0 KB /usr/bin/sort 0 KB /usr/bin/ssh 0 KB /usr/bin/ssh 0 KB /usr/bin/ssh-agent 0 KB /usr/libexec/postfix/pickup 0 KB /usr/libexec/postfix/qmgr 0 KB /usr/sbin/atd 0 KB /usr/sbin/dnsmasq 0 KB /usr/sbin/dnsmasq 0 KB /usr/sbin/sedispatch 0 KB /usr/sbin/sshd 0 KB /usr/sbin/sshd 28 KB /usr/sbin/chronyd 32 KB /usr/sbin/audispd 84 KB /usr/sbin/avahi-daemon 88 KB /usr/lib/systemd/systemd-logind 100 KB /usr/bin/tail 104 KB /usr/sbin/crond 156 KB /usr/sbin/avahi-daemon 192 KB /usr/lib/systemd/systemd-journald 196 KB /usr/bin/bash 196 KB /usr/bin/dbus-launch ... 14872 KB /usr/bin/Xvnc 20048 KB /usr/lib64/firefox/firefox 40176 KB /usr/lib64/firefox/firefox 108848 KB /usr/sbin/mysqld 267144 KB Total

This actually says mysql is using the most swap.

$ free -k total used free shared buff/cache available Mem: 1883740 1044212 112132 14320 727396 520304 Swap: 2097148 265784 1831364

free's ouput doesn't seem too much off in my case

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    Your command gave exactly the same result as mine (using status or smaps didn't show a real difference). Alas, your answer doesn't help me to understand what is using my swap space. :( Commented Jul 17, 2018 at 15:21
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    I am pretty much sure this answers "How can I know which process is using my swap space on my Linux system?", the answer for my system is mysqld. Maybe you should reformulate the question, if you are asking something diffrent? Commented Jul 17, 2018 at 16:52
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    "What is consuming 244 MB of swap?" would be a more precise question in my case. I want to know where those MB are consumed. I will update my question to be more specific. Commented Jul 23, 2018 at 11:58
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This will print each PID that currently use swap memory:

for pid in $(ls /proc/ | grep -Po '^\d+$'); do echo -n "$pid "; cat /proc/$pid/status | grep VmSwap; done | grep -vE ' 0 kB$' | grep -Eo '[0-9]+ VmSwap:.+' | sed -r 's/VmSwap://g'
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    ... and all processes whose name simply contains swap, as in kswapd. Commented Oct 21, 2021 at 8:17

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