Limited docker exec concurreny

[mblaschke]

I'm using serverspec to test docker images (executing shell commands via exec ) and running each image in a parallel test. If i execute more then 6 parallel tests these tests are getting slower and slower.
Also docker stats isn't reliable anymore because all values are displayed as dashes instead of values.

Serverspec is using the docker-api gem.

I've tried to build a small quick&dirty testcase and gatherd as much information i was able to get:
https://github.com/mblaschke/docker-api-stresstest
docker info, docker version and server specs are included.

Maybe it's related to #13885 but the Docker daemon is still responding. But it's still limiting the execution requests. While testing there were no cashes or other problems just a limited concurrency to about 20 executions per second with a very low system load on the machine.

[thaJeztah]

ping @mlaventure @crosbymichael @unclejack PTAL