How many CPUs does my docker container have?

Question

I'm writing a library that operates in parallel. This library is often used within docker containers. I would like to start as many threads as my docker container has allocated cores.

Does docker set the CPUs limit somewhere as an environment variable?

For example if my user sets two CPUs when creating the container:

docker run --cpuset-cpus="2" myapp:latest

(see this question)

How do I get back the number 2 from within the container by inspecting the container's state?

Answer 1

With --cpuset-cpus="2" you actually use 1 cpu. For example, if you have 4 available: {0,1,2,3} you have to specify 2 of them by separating them with comma or by defining a range. From the docs:

--cpuset-cpus

Limit the specific CPUs or cores a container can use. A comma-separated list or hyphen-separated range of CPUs a container can use, if you have more than one CPU. The first CPU is numbered 0. A valid value might be 0-3 (to use the first, second, third, and fourth CPU) or 1,3 (to use the second and fourth CPU).

Answer:

• To get the number of processing units available: nproc
• To get the cpu set: /sys/fs/cgroup/cpuset/cpuset.cpus

small example follows:

1. host:

   $ nproc
   4
   

2. container with 1 cpu:

   $ docker run --rm -it --cpuset-cpus="2" ubuntu
   root@73844de506db:/# nproc
   1
   root@73844de506db:/# cat /sys/fs/cgroup/cpuset/cpuset.cpus
   2
   

3. container with 3 cpus:

   $ docker run --rm -it --cpuset-cpus="0-2" ubuntu
   root@4c3f841e613b:/# nproc
   3
   root@4c3f841e613b:/# cat /sys/fs/cgroup/cpuset/cpuset.cpus
   0-2