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Memory limits

By default, a container has no resource constraints and can use as much of a given resource as the host’s kernel scheduler allows. Docker provides ways to control how much memory a container can use, setting runtime configuration flags of the docker run command.

If a capability is disabled in your kernel, you may see a warning at the end of the output like the following:

WARNING: No swap limit support

Prerequisites

Having completed lab 00 - Setup lab environment

Connect to Sunnyvale's Ubuntu VM

$ cd <GIT_REPO_NAME>/vagrant
$ vagrant up
$ vagrant ssh
vagrant@docker-vm:~$

Test Linux support for memory limits

Test Docker support for memory limits

vagrant@docker-vm:~$ docker info
...
WARNING: No swap limit support

Edit file /etc/default/grub.d/50-cloudimg-settings.cfg to add cgroup_enable=memory swapaccount=1 flags. This will enable Linux kernel to support memory limits imposed by Docker.

vagrant@docker-vm:~$ sudo perl -p -i  -e "s/GRUB_CMDLINE_LINUX_DEFAULT=\"console=tty1 console=ttyS0\"/GRUB_CMDLINE_LINUX_DEFAULT=\"console=tty1 console=ttyS0 cgroup_enable=memory swapaccount=1\"/g" /etc/default/grub.d/50-cloudimg-settings.cfg

Apply the change and reboot

vagrant@docker-vm:~$ sudo update-grub && sudo reboot
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.4.0-146-generic
Found initrd image: /boot/initrd.img-4.4.0-146-generic
done
Connection to 127.0.0.1 closed by remote host.
Connection to 127.0.0.1 closed.

Test with docker info again

vagrant@docker-vm:~$ docker info
...

No warning is displayed at the end of the output, Linux is supporting Docker limits on memory as expected.

Use Docler memory limits

Get the Docker Host (Ubuntu VM in this case) total memory available:

vagrant@docker-vm:~$ cat /proc/meminfo | grep MemTotal
MemTotal:        2045912 kB

Run a container to test the Max Memory seen by a Java application

vagrant@docker-vm:~$ docker run \
    --rm \
    -v /home/vagrant/$GIT_REPO_NAME/labs/15-Memory_limits/app:/app/ \
    openjdk:11.0-jdk \
    /usr/local/openjdk-11/bin/java \
    -XX:MaxRAMFraction=1 \
    /app/MaxMemory.java
Max Memory: 1998

Run a container with --memory 500m flag (1/4 of the host's memory)

vagrant@docker-vm:~$ docker run \
    --rm \
    --memory 500m \
    -v /home/vagrant/$GIT_REPO_NAME/labs/15-Memory_limits/app:/app/ \
    openjdk:11.0-jdk \
    /usr/local/openjdk-11/bin/java \
    -XX:MaxRAMFraction=1 \
    /app/MaxMemory.java
Max Memory: 483