Docker Swap Limit – WARNING: No Swap Limit Support

dockerswap

I'm running Docker(1.9.1) on Ubuntu 16.04. When I run docker info the last line of the output says WARNING: No swap limit support.

INFO[0781] GET /v1.21/info                              
Containers: 0
Images: 0
Server Version: 1.9.1
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 0
 Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 4.4.0-62-generic
Operating System: Ubuntu 16.04.1 LTS (containerized)
CPUs: 2
Total Memory: 3.664 GiB
Name: lenovo
ID: A3ZV:2EVK:U5QB:O7CG:PEDL:SANK:X74X:QNLC:VOTK:GFDR:S24T:C5KT
WARNING: No swap limit support

What does this warning mean? I definitely have a swap partition, as evidenced by free -mh though I don't understand why my swap has no entry under available

total        used        free      shared  buff/cache   available
Mem:           3.7G        1.9G        182M        157M        1.6G           1.3G
Swap:          3.8G        2.9M        3.8G

Best Answer

Swap limit support allows you to limit the swap the container uses, see https://docs.docker.com/engine/admin/resource_constraints

According to https://docs.docker.com/engine/installation/linux/linux-postinstall/#your-kernel-does-not-support-cgroup-swap-limit-capabilities :

You can enable these capabilities on Ubuntu or Debian by following these instructions. Memory and swap accounting incur an overhead of about 1% of the total available memory and a 10% overall performance degradation, even if Docker is not running.

1) Log into the Ubuntu or Debian host as a user with sudo privileges.

2) Edit the /etc/default/grub file. Add or edit the GRUB_CMDLINE_LINUX line to add the following two key-value pairs:
GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"
3) Update GRUB.
$ sudo update-grub

System V shared memory

Finally it's possible you have some horrible legacy software that uses system V shared memory segments. If they get leaked, they won't show up in top :(.

You can list them with ipcs -m -t. Hopefully the most recently created one is still in use. Take the shmid number and e.g.

$ ipcs -m -t

------ Shared Memory Attach/Detach/Change Times --------
shmid      owner      attached             detached             changed             
3538944    alan       Apr 30 20:35:15      Apr 30 20:35:15      Apr 30 16:07:41     
3145729    alan       Apr 30 20:35:15      Apr 30 20:35:15      Apr 30 15:04:09     
4587522    alan       Apr 30 20:37:38      Not set              Apr 30 20:37:38     

# sudo grep 4587522 /proc/*/maps

-> then the numbers shown in the /proc paths are the pid of the processes that use the SHM. (So you could e.g. grep the output of ps for that pid number).

Apparent contradictions

Xorg has 8G mapped. Even though you don't have separate video card RAM. It only has 150M resident. It's not that the rest is swapped out, because you don't have enough swap space.
The SHM segments shown by ipcs are all attached to two processes. So none of them have leaked, and they should all show up in the SHR column of top (double-counted even). It's ok if the number of pages used is less than the size of the memory segment, that just means there are pages that haven't been used. But free says we have 6GB of allocated shared memory to account for, and we can't find that.

Fedora – Docker Firewalld/iptables WARNING: COMMAND_FAILED

This is more like a warning that these rules already exists. Because the error message is missing it is not a real error.

See https://github.com/moby/moby/issues/16137

Best Answer

Related Solutions

Linux Swap – System Becomes Unresponsive Despite Free RAM

System V shared memory

Apparent contradictions

Fedora – Docker Firewalld/iptables WARNING: COMMAND_FAILED

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