Container runtimes

One of the terms you hear a lot when dealing with containers is “container runtime”. This post will explain what container runtimes are.


Docker, Google, CoreOS, and other vendors created the Open Container Initiative (OCI). The OCI currently contains two specifications: the Runtime Specification (runtime-spec) as a standerd of CRI (Container runtime Interface) and the Image Specification (image-spec). This led to other standards like CNI (Container Network Interface), a Cloud Native Computing Foundation project, or Container Storage Interface (CSI).

What is a container runtime?

Container runtime is the engine that runs and manages the components required to run containers. Communicating with the kernel to start containerized processes, setting up cgroups, configure mount points and do many things to make your container work.


Docker was released in 2013 and solved many of the problems that developers had running containers like LXC or OpenVZ. Before version 1.11, the implementation of Docker was a monolithic daemon. The monolith did everything as one package such as downloading container images, launching container processes, exposing a remote API, and acting as a log collection daemon, all in a centralized process running as root. (Source: Coreos )

At this time docker was the only runtime that Kubernetes supported, but wit the release of Coreos’s rkt Kubernetes needed a standard interface to ease the integration of other container runtimes. This led to the splitting of Docker into different parts.

Example image

Example image


CRI-O is an implementation of the Kubernetes CRI (Container Runtime Interface) to enable using OCI (Open Container Initiative) compatible runtimes. It is a lightweight alternative to using Docker as the runtime for kubernetes. It allows Kubernetes to use any OCI-compliant runtime as the container runtime for running pods. Today it supports runc and Kata Containers as the container runtimes but any OCI-conformant runtime can be plugged in principle.

CRI-O supports OCI container images and can pull from any container registry. It is a lightweight alternative to using Docker, Moby or rkt as the runtime for Kubernetes. (Source: CRI-O Website )

crictl ps - list containers
crictl pods - list pods


PouchContainer is an open-source project created by Alibaba Group. It provides applications with a lightweight runtime environment with strong isolation and minimal overhead. PouchContainer isolates applications from varying runtime environment, and minimizes operational workload. t includes lots of security features, like hypervisor-based container technology, lxcfs, directory disk quota, patched Linux kernel. PouchContainer utilizes Dragonfly, a P2P-base distribution system, to achieve lightning-fast container image distribution at enterprise’s large scale.

Kata Containers

Kata Containers is an open source community working to build a secure container runtime with lightweight virtual machines that feel and perform like containers, but provide stronger workload isolation using hardware virtualization technology as a second layer of defense. (Source: Kata Containers Website )


Podman is a daemonless, open source, Linux-native tool designed to develop, manage, and run Open Container Initiative (OCI) containers and pods. It has a similar directory structure to Buildah, Skopeo, and CRI-O. Podman doesn’t admin privileges for its commands to work.


containerd is a daemon that controls runC. From containerd website, “containerd manages the complete container lifecycle of its host system, from image transfer and storage to container execution and supervision to low-level storage to network attachments and beyond”.

What is a Low-Level Container Runtime?

Containers are implemented using Linux namespaces and cgroups. Namespaces let you virtualize system resources, like the file system or networking, for each container. Cgroups provide a way to limit the amount of resources like CPU and memory that each container can use. At the lowest level, container runtimes are responsible for setting up these namespaces and cgroups for containers, and then running commands inside those namespaces and cgroups. Low-level runtimes support using these operating system features.


runc is a CLI tool for spawning and running containers according to the OCI specification. Docker donated this library to OCI as a reference implementation of the OCI runtime specification.


crun is a lightweight fully featured OCI runtime and C library for running containers.

“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.” (Source: crun GitHub page )

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

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


runv is a hypervisor-based runtime for OCI. runV supports the following hypervisors:

  • KVM (QEMU 2.1 or later)
  • KVM (Kvmtool)
  • Xen (4.5 or later)
  • QEMU without KVM (NOT RECOMMENDED. QEMU 2.1 or later)