how to kill lots of docker container processes effectively and faster?

Try this:

1. Uninstall docker-engine
2. Reboot host
3. rm /var/lib/docker

Rebooting effectively stops all of the containers and uninstalling docker prevents them from coming back upon reboot. (in case they have restart=always set)

If you are interesting in only killing the processes as they are not exiting properly (my assessment of what you mean--correct me if I'm wrong), there is a way to walk the running container processes and kill them using the Pid information from the container's metadata. As it appears you don't necessarily care about clean process shutdown at this point (which is why docker kill is taking so long per container--the container may not respond to the right signals and therefore the engine waits patiently, and then kills the process), then a kill -9 is a much more swift and drastic way to end these containers and clean up.

A quick test using the latest docker release shows I can kill ~100 containers in 11.5 seconds on a relatively modern laptop:

$ time docker ps --no-trunc --format '{{.ID}}' | xargs -n 1 docker inspect --format '{{.State.Pid}}' $1 | xargs -n 1 sudo kill -9real    0m11.584suser    0m2.844ssys     0m0.436s

A clear explanation of what's happening:

1. I'm asking the docker engine for an "full container ID only" list of all running containers (the docker ps)
2. I'm passing that through docker inspect one by one, asking to output only the process ID (.State.Pid), which
3. I then pass to the kill -9 to have the system directly kill the container process; much quicker than waiting for the engine to do so.

Again, this is not recommended for general use as it does not allow for standard (clean) exit processing for the containerized process, but in your case it sounds like that is not important criteria.

If there is leftover container metadata for these exited containers you can clean that out by using:

docker rm $(docker ps -q -a --filter status=exited)

This will remove all exited containers from the engine's metadata store (the /var/lib/docker content) and should be relatively quick per container.

So,

docker kill $(docker ps -a -q)

isn't what you need?

EDIT: obviously it isn't. My next take then:

A) somehow create a list of all containers that you want to stop.

B) Partition that list (maybe by just slicing it into n parts).

C) Kick of n jobs in parallel, each one working one of those list-slices.

D) Hope that "docker" is robust enough to handle n processes sending n kill requests in sequence in parallel.

E) If that really works: maybe start experimenting to determine the optimum setting for n.