I have two containers ran by Docker Swarm:
# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
18f1e7d79f5b default/image1:latest "/bin/sh -c 'java ..." 12 hours ago Up 12 hours image1.1.x56zz152kmqtqpxzle5nkbs8r
11d1f05fcff1 default/image2:latest "java -cp /app/sca..." 13 hours ago Up 13 hours image2.1.ljztzeeh8i5r6ebr3n4hcj45e
$ docker stats --no-stream
CONTAINER CPU % MEM USAGE / LIMIT MEM % NET I/O BLOCK I/O PIDS
18f1e7d79f5b 0.92% 0B / 0B 0.00% 648B / 0B 0B / 0B 0
11d1f05fcff1 0.91% 0B / 0B 0.00% 39.9MB / 26.9MB 51.2MB / 0B 0
Host OS is Debian Jessie. There are JVM applications in both containers. Why does docker stats show zero memory usage? Should I configure my Java apps somehow to be able to see memory usage in docker stats? Is there some other way to check memory usage?
It is quite possible that memory cgroup accounting is not enabled in your kernel (which is a common problem that others have hit as well over time on Debian/Ubuntu based installs).
Can you verify with docker info or the output from the daemon that you aren't seeing messages like the following, nicely summarized in this blog post: Debian Jessie w/Docker? If so, the fix is also listed there, as well as in the GitHub issue I linked in the first sentence. There is also a section in the install documentation about enabling cgroup memory accounting although it only mentions Ubuntu and not Debian.