docker swarm container memory limit does not work

Question

I'm experimenting with forcing a container to use more memory than it's allowed but I can't get it to work. The container is part of a stack defined with docker compose and it's deployed to docker in swarm mode.

Docker is allowing the container to go way above the 50M limit I've set. I was expecting docker to kill the container, throw an error, etc.

Can anyone help me on why Docker does not enforce the memory limit here?

The container in docker-compose.yml is defined to have a memory limit of 50M, and then I have setup a very simple PHP test which will try to allocate 200M. I've defined PHP mem limit to 128M.

This is my docker-compose.yml

  version: "3"

  services:
    nginx:
      image: nginx:latest
      restart: unless-stopped
      volumes:
        - ./deploy/nginx/nginx.conf:/etc/nginx/nginx.conf
        - ./public:/usr/share/nginx/html
      ports:
        - "8180:80"
      links:
        - app

    app:
      image: 127.0.0.1:5000/wpdemo
      build:
        context: .
        dockerfile: Dockerfile-app
      restart: unless-stopped
      volumes:
        - .:/var/www/html
      links:
        - mysql
      deploy:
        resources:
          limits:
            cpus: '0.50'
            memory: 50M
          reservations:
            cpus: '0.25'
            memory: 20M

    mysql:
        image: mysql:5.7
        restart: unless-stopped
        ports:
            - "13306:3306"
        environment:
            MYSQL_ALLOW_EMPTY_PASSWORD: 'yes'
        volumes:
            - ~/docker/volumes/mysql:/var/lib/mysql

Instead of docker killing the container, it allows it to take as much memory as it wants and PHP eventually stops the process throwing the error below:

"PHP message: PHP Fatal error: Allowed memory size of 125829120 bytes exhausted (tried to allocate 67108872 bytes) in /var/www/html/public/index.php on line 4"

I'm using Ubuntu 18.04.

uname -a
Linux  4.18.10-041810-generic #201809260332 SMP Wed Sep 26 07:34:01 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Docker version 18.06.1-ce, build e68fc7a

docker-compose version 1.17.1, build unknown docker-py version: 2.5.1 CPython version: 2.7.15rc1 OpenSSL version: OpenSSL 1.1.0g 2 Nov 2017

This is the output of "docker stats" on the app container:

CONTAINER ID        NAME                                        CPU %               MEM USAGE / LIMIT   MEM %               NET I/O             BLOCK I/O           PIDS
679c8495ac1d        stackdemo_app.1.hr3ufwlskhdafre39aqrshxyu   0.00%               43.81MiB / 50MiB    87.62%              106kB / 389kB       2.05GB / 10.6GB     5

This is the output of "docker info":

Containers: 36
 Running: 5
 Paused: 0
 Stopped: 31
Images: 450
Server Version: 18.06.1-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: active
 NodeID: wnegv5lp41wfs3epfrua489or
 Is Manager: true
 ClusterID: hq7o176yffjglxzb9pu3fiomr
 Managers: 1
 Nodes: 1
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 10
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address: 192.168.1.120
 Manager Addresses:
  192.168.1.120:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 468a545b9edcd5932818eb9de8e72413e616e86e
runc version: 69663f0bd4b60df09991c08812a60108003fa340
init version: fec3683
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.18.10-041810-generic
Operating System: Ubuntu 18.04.1 LTS
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 15.49GiB
Name: rafxps15
ID: QEX7:FEB3:J76L:DCAQ:SO4S:SWVE:4XPI:PI6R:YM4C:MV4I:C3PM:FLOQ
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

WARNING: No swap limit support

Answer 1

As you said in comment, swap is enabled on host but swap limit in cgroups does not supported yet.

According to this enabling swap limit support . Note that reboot of system is essential.

At last, —-memory-swap flag should be set. If you want to prevent your PHP app accessing swap, you should set it with the same value of —-memory . More details about memory swap settings .