Why does RHEL use swap even when vm.swappiness = 1?

Question

RHEL 7.2 memory use, per free -m:

              total        used        free      shared  buff/cache   available
Mem:         386564       77941       57186         687      251435      306557
Swap:         13383        2936       16381

we see that used swap is 2936M

so we want to decrease it to min by the following

echo 1 > /proc/sys/vm/swappiness

sysctl -w vm.swappiness=1

echo "vm.swappiness = 1" >> /etc/sysctl.conf

and after 10 min we check again , but still OS used the swap

free -m:

              total        used        free      shared  buff/cache   available
Mem:         386564       77941       57186         687      251435      306557
Swap:         13389        2930       16381

why the actions that we did not take affect immeditly?

Do we need to restart the OS, in order to get swap used to be 0 ?

example

we run vmstat:

 vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 3  0  85740 20255872 2238248 183126400    0    0     7   162    0    0  7  1 92  0  0

we decrease the vm.swappiness=1

and run vmstat after 10min:

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 3  0  85740 20255872 2238248 183126400    0    0     7   162    0    0  7  1 92  0  0

Answer 1

As you’ve been told before (see Why does swappiness not work?), changing swappiness only affects future decisions made by the kernel when it needs to free memory. Reducing it won’t cause the kernel to reload everything that’s been swapped out.

Your vmstat output shows that swap isn’t being actively used, i.e. your current workloads really don’t need the pages which have been swapped out.

There’s no point in trying to micro-manage the kernel’s use of swap in the way you tend to do. Depending on your workload, decide whether you need to favour the page cache or not, adjust swappiness accordingly, then leave the system to run.

If you really want to clear swap, disable it and re-enable it:

swapoff -a && swapon -a

Answer 2

free -m is not a reliable source of information about swap use. Instead, please use vmstat before and after the echo commands which temporarily change swappiness.

1) swapoff -a && swapon -a && vmstat
2) do work which requires swapping
3) vmstat

Now you know how much swapping is going on before changing the swappiness. If there's no swapping going on, find other jobs which do swap.

4) use and echo command to change swappiness temporarily
5) swapoff -a && swapon -a && vmstat
6) do work which requires swapping
7) vmstat

8) compare the si and so values.

The values to watch are:

si: Amount of memory swapped in from disk (/s).
so: Amount of memory swapped to disk (/s).

You may also find helpful information in the RHEL 7 Performance Tuning Guide.

Many thanks to Stephen Kitt for reminding me about swapon and swapoff.