You may get an error "kernel does not support cgroup swap limit Docker" while starting the Docker server on Ubuntu 

​# docker -d

2015/05/13 14:37:48 docker daemon: 1.3.0 c78088f; execdriver: native;
[e6c7ea54] +job serveapi(unix:///var/run/docker.sock)
[info] Listening for HTTP on unix (/var/run/docker.sock)
[e6c7ea54] +job init_networkdriver()
[e6c7ea54] -job init_networkdriver() = OK (0)
2015/05/13 14:37:48 WARNING: Your kernel does not support cgroup swap limit.
[info] Loading containers:


We need to ensure the supporting package "apparmor" has installed in your server.
​# apt-get install apparmor

Now, start the docker service.
# service docker start
​docker start/running, process 7021

You can check whether the docker service is running or not by ps command.
 # ps aux | grep docker

root      7021  1.0  0.6 330428 11044 ?        Ssl  14:41   0:00 /usr/bin/docker -d
root      7054  0.0  0.0  15940   920 pts/16   S+   14:41   0:00 grep --color=auto docker