crun

A fast and low-memory footprint OCI Container Runtime fully written in C.

crun conforms to the OCI Container Runtime specifications (https://github.com/opencontainers/runtime-spec).

Documentation

The user documentation is available here.

Why another implementation?

While most of the tools used in the Linux containers ecosystem are written in Go, I believe C is a better fit for a lower level tool like a container runtime. runc, the most used implementation of the OCI runtime specs written in Go, re-execs itself and use a module written in C for setting up the environment before the container process starts.

crun aims to be also usable as a library that can be easily included in programs without requiring an external process for managing OCI containers.

Performance

crun is faster than runc and has a much lower memory footprint.

This is the elapsed time on my machine for running sequentially 100 containers, the containers run /bin/true:

	crun	runc	%
100 /bin/true	0:01.69	0:3.34	-49.4%

crun requires fewer resources, so it is also possible to set stricter limits on the memory and number of PIDs allowed in the container:

# podman --runtime /usr/bin/runc run --rm --pids-limit 1 fedora echo it works
Error: container_linux.go:346: starting container process caused "process_linux.go:319: getting the final child's pid from pipe caused \"EOF\"": OCI runtime error

# podman --runtime /usr/bin/crun run --rm --pids-limit 1 fedora echo it works
it works

# podman --runtime /usr/bin/runc run --rm --memory 4M fedora echo it works
Error: container_linux.go:346: starting container process caused "process_linux.go:327: getting pipe fds for pid 13859 caused \"readlink /proc/13859/fd/0: no such file or directory\"": OCI runtime command not found error

# podman --runtime /usr/bin/crun run --rm --memory 4M fedora echo it works
it works

crun could go much lower than that, and require < 1M. The used 4MB is a hard limit set directly in Podman before calling the OCI runtime.

Dependencies

These dependencies are required for the build:

Fedora

dnf install -y make python git gcc automake autoconf libcap-devel \
    systemd-devel yajl-devel libseccomp-devel \
    go-md2man glibc-static python3-libmount libtool

RHEL/CentOS 8

yum --enablerepo='*' install -y make automake autoconf gettext \
    libtool gcc libcap-devel systemd-devel yajl-devel \
    libseccomp-devel python36 libtool

go-md2man is not available on RHEL/CentOS 8, so if you'd like to build the man page, you also need to manually install go-md2man. It can be installed with:

yum --enablerepo='*' install -y golang
export GOPATH=$HOME/go
go get github.com/cpuguy83/go-md2man
export PATH=$PATH:$GOPATH/bin

Ubuntu

apt-get install -y make git gcc build-essential pkgconf libtool \
   libsystemd-dev libcap-dev libseccomp-dev libyajl-dev \
   go-md2man libtool autoconf python3 automake

Alpine

apk add gcc automake autoconf libtool gettext pkgconf git make musl-dev \
    python3 libcap-dev libseccomp-dev yajl-dev argp-standalone go-md2man

Build

Unless you are also building the Python bindings, Python is needed only by libocispec to generate the C parser at build time, it won't be used afterwards.

Once all the dependencies are installed:

./autogen.sh
./configure
make

To install into default PREFIX (/usr/local):

sudo make install

Static build

It is possible to build a statically linked binary of crun by using the officially provided nix package and the derivation of it within this repository. The builds are completely reproducible and will create a x86_64/amd64 stripped ELF binary for glibc.

Nix

To build the binaries by locally installing the nix package manager:

nix build -f nix/

Ansible

An Ansible Role is also available to automate the installation of the above statically linked binary on its supported OS:

sudo su -
mkdir -p ~/.ansible/roles
cd ~/.ansible/roles
git clone https://github.com/alvistack/ansible-role-crun.git crun
cd ~/.ansible/roles/crun
pip3 install --upgrade --ignore-installed --requirement requirements.txt
molecule converge
molecule verify