CentOS – Concern About Swap Usage with Nearly 40GB of Free Memory

This is not a problem and is likely normal. Lots of code (and possibly data) is used very rarely so the system will swap it out to free up memory.

Swapping is mostly only a problem if memory is being swapped in and out continuously. It is that kind of activity that kills performance and suggests a problem elsewhere on the system.

If you want to monitor your swap activity you can with several utilities but vmstat is usually quite useful e.g.

$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 348256  73540 274600    0    0     1     9    9    6  2  0 98  0  0
 0  0      0 348240  73544 274620    0    0     0    16   28   26  0  0 100  0  0
 0  0      0 348240  73544 274620    0    0     0     0   29   33  0  0 100  0  0
 0  0      0 348240  73544 274620    0    0     0     0   21   23  0  0 100  0  0
 0  0      0 348240  73544 274620    0    0     0     0   24   26  0  0 100  0  0
 0  0      0 348240  73544 274620    0    0     0     0   23   23  0  0 100  0  0

Ignore the first line as that is activity since the system started. Note the si and so columns under ---swap--; they should generally be fairly small figures if not 0 for the majority of the time.

Also worth mentioning is that this preemptive swapping can be controlled with a kernel setting. The file at /proc/sys/vm/swappiness contains a number between 0 and 100 that tells the kernel how aggressively to swap out memory. Cat the file to see what this is set to. By default, most Linux distros default this to 60, but if you don't want to see any swapping before memory is exhausted, echo a 0 into the file like this:

echo 0 >/proc/sys/vm/swappiness

This can be made permanent by adding

vm.swappiness = 0

to /etc/sysctl.conf.