30 September 2017

Fix slow container shutdown

Tags: Docker

Today I was working in a dev environment where everything was done within containers and docker-compose was used to define the services. Usually, when I work on my own projects in this way I don't have to rebuild containers every time I make changes to my code to reflect those changes but in this special case, there was no getting around it. Every time I made changes to the code I had to rebuild the container and run it again. The annoying part was that it took such a long time to re-up the container because the stop process took such a long time. I investigated it and it seems I have been running my apps in the container as the PID 1 process. How does this happen?

There are two ways of running your containers. With CMD or ENTRYPOINT or a combination of both. For example

 

FROM alpine:3.6

CMD ["ping"]

and

FROM alpine:3.6
ENTRYPOINT ["ping"]
CMD ["8.8.8.8"]

are two ways of defining how your container should later be run. 

CMD lets the user that runs the container overwrite the command where ENTRYPOINT does not give this freedom thus making the container a ping only app.

In both cases running the containers, the ping application is run as PID 1 and comes with a bunch of responsibilities and edge cases that we have not anticipated - notably the handling of SIGTERMs.

Lets have a look at the container

docker run --rm -it --name pinger donchev7/alpine-pinger
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: seq=0 ttl=59 time=10.643 ms
64 bytes from 8.8.8.8: seq=1 ttl=59 time=9.861 ms
64 bytes from 8.8.8.8: seq=2 ttl=59 time=12.448 ms

The donchev7/alpine-pinger image is only 3.97MB, you should have no problem running this container in seconds.

While the container is running issue this command in another terminal window

docker exec pinger ps aux

you should see the following

docker exec pinger ps aux                               
PID   USER     TIME   COMMAND
1     root     0:00   ping 8.8.8.8
7     root     0:00   ps aux

There it is, ping is PID 1

Connecting the dots

When we issue a docker stop command the docker daemon sends a SIGTERM signal to stop our pinger but since it's PID 1 nothing happens and after a while (10s) the docker daemon loses its patience with the container and sends a SIGKILL. Why does this happen? Obviously, because our application running in the container wasn't programmed to listen to SIGTERM.
While still having your pinger container running run the following

time docker stop pinger

On my computer it took 10,577 seconds to stop the container. That's crazy, containers should be lighting fast :)

Solutions

  • Add STOPSIGNAL SIGINT  to your Dockerfile (this is a quick & dirty solution and only takes care of one edge case and shouldn't actually be used

  • Avoid being PID 1 when you run ad-hoc docker run commands you can pass the--init flagand your app won't become PID 1

Let's test the second solution out. Run the pinger again this time passing the --init flag like so

docker run --rm -it --name pinger --init donchev7/alpine-pinger

and then

time docker stop pinger

Now it takes around 500ms to stop the running container!

BONUS

If you are using docker-compose and a YML version 2.2 or 2.3 you can do the same

version: '2.3'
services:
  pinger:
    image: donchev7/alpine-pinger
    init: true

save this in a docker-compose.yml file and your docker-compose stop command should be lightning fast!