Find out what is using your swap

Article by Erik Ljungstrom, published on

Have you ever logged in to a server, ran `free`, seen that a bit of swap is used and wondered what’s in there? It’s usually not very indicative of anything, or even overly helpful knowing what’s in there, mostly it’s a curiosity thing.

Either way, starting from kernel 2.6.16, we can find out using smaps which can be found in the proc filesystem. I’ve written a simple bash script which prints out all running processes and their swap usage.
It’s quick and dirty, but does the job and can easily be modified to work on any info exposed in /proc/$PID/smaps
If I find the time and inspiration, I might tidy it up and extend it a bit to cover some more alternatives. The output is in kilobytes.

# Get current swap usage for all running processes
# Erik Ljungstrom 27/05/2011
for DIR in `find /proc/ -maxdepth 1 -type d | egrep "^/proc/[0-9]"` ; do
PID=`echo $DIR | cut -d / -f 3`
PROGNAME=`ps -p $PID -o comm --no-headers`
for SWAP in `grep Swap $DIR/smaps 2>/dev/null| awk '{ print $2 }'`
echo "PID=$PID - Swap used: $SUM - ($PROGNAME )"

echo "Overall swap used: $OVERALL"

This will need to be ran as root for it to be able to gather accurate numbers. It will still work even if you don’t, but it will report 0 for any processes not owned by your user.
Needless to say, it’s Linux only. The output is ordered alphabetically according to your locale (which admittedly isn’t a great thing since we’re dealing with numbers), but you can easily apply your standard shell magic to the output. For instance, to find the process with most swap used, just run the script like so:

$ ./ | sort -n -k 5
Don’t want to see stuff that’s not using swap at all?
$ ./ | egrep -v "Swap used: 0" |sort -n -k 5

… and so on and so forth

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