docker swarm container memory limit does not work - docker

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.