Viewing Docker Logs

• Docker container logs can be viewed by using the command docker logs <container>
• Lets experiment by executing the jenkins container

docker container run --name jenkins -d -P jenkins

• Now execute the following command

docker container logs jenkins

• Docker Swarm service or task logs can be viewed by using the docker service logs
• The above two commands shows the command output redirected to STDOUT and STDERR streams
• In some cases the logs of your application might be redirected to the file or an external databases and the above commands will not be useful.
• Majority of the users while creating Docker Image redirect their logs to /dev/stdout. (which is a best practice)
• However docker includes multiple logging mechanisms to get info from running services & containers. These mechanisms are referred as logging drivers.
• Each Docker daemon has a default loggin driver, which every container uses, if the logging driver is not explicitly configured.
• Docker give the options to extend/implement/install logging dirivers. Reference for more info.
• For the complete list of supported logging drivers click here

Resource Constraints

• By default, container has no resource restrictions & it can use as much of a given resource as the host’s kernel allows.
• On Docker container memory, cpu and GPU constraints can be imposed using docker run or docker container run command

Lets start experimentation with following containers

• In these commands we generally follow number by suffix of b,k,m,g to indicate bytes, kilobytes, megabytes, gigabytes respectively.
• Lets create a jenkins container with memory restriction of 512 MB and one without it.

docker container run --name r-mem-jenkins -P -d --memory 512m jenkins
docker container run --name jenkins -P -d jenkins

• Execute docker stats and you should see the output as

CONTAINER ID        NAME                CPU %               MEM USAGE / LIMIT MEM %               NET I/O             BLOCK I/O           PIDS

f3ba989220d7        r-mem-jenkins       0.07%               489.8MiB / 512MiB     95.66%              4.01MB / 19.5kB     2.22MB / 23.5MB     34

6cbfdf4ca307        jenkins             0.12%               499.3MiB / 3.794GiB   12.85%              4.01MB / 23kB       0B / 23.5MB         34


## Specific Area of interest
NAME             MEM USAGE / LIMIT
r-mem-jenkins    489.8MiB / 512MiB
jenkins          499.3MiB / 3.794GiB

• similarly the other options like

  • –memory-swap
  • –memory-swappiness
  • –memory-reservation

• Similarly we can impose restrictions on cpu’s as well. In the following example we would create two containers, in which one container to get a guaranteed to get at the most one cpu"

docker container run --name r-memcpu-jenkins -P -d --cpus="1" --memory 512m jenkins
docker container run --name jenkins -P -d jenkins

• Execute docker stats and observe the output
• There are other command line options for GPU as well.
• Execute docker container run --help to explore all the other options available for restrictions