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Virtual machine vCPU sizing in Kata Containers

Default number of virtual CPUs

Before starting a container, the runtime reads the default_vcpus option from the configuration file to determine the number of virtual CPUs (vCPUs) needed to start the virtual machine. By default, default_vcpus is equal to 1 for fast boot time and a small memory footprint per virtual machine. Be aware that increasing this value negatively impacts the virtual machine's boot time and memory footprint. In general, we recommend that you do not edit this variable, unless you know what are you doing. If your container needs more than one vCPU, use Kubernetes cpu limits to assign more resources.

Kubernetes

# ~/cpu-demo.yaml
apiVersion: v1
kind: Pod
metadata:
  name: cpu-demo
  namespace: sandbox
spec:
  containers:
  - name: cpu0
    image: vish/stress
    resources:
      limits:
        cpu: "3"
    args:
    - -cpus
    - "5"
$ sudo -E kubectl create -f ~/cpu-demo.yaml

Virtual CPUs and Kubernetes pods

A Kubernetes pod is a group of one or more containers, with shared storage and network, and a specification for how to run the containers [specification]. In Kata Containers this group of containers, which is called a sandbox, runs inside the same virtual machine. If you do not specify a CPU constraint, the runtime does not add more vCPUs and the container is not placed inside a CPU cgroup. Instead, the container uses the number of vCPUs specified by default_vcpus and shares these resources with other containers in the same situation (without a CPU constraint).

Container lifecycle

When you create a container with a CPU constraint, the runtime adds the number of vCPUs required by the container. Similarly, when the container terminates, the runtime removes these resources.

Container without CPU constraint

A container without a CPU constraint uses the default number of vCPUs specified in the configuration file. In the case of Kubernetes pods, containers without a CPU constraint use and share between them the default number of vCPUs. For example, if default_vcpus is equal to 1 and you have 2 containers without CPU constraints with each container trying to consume 100% of vCPU, the resources divide in two parts, 50% of vCPU for each container because your virtual machine does not have enough resources to satisfy containers needs. If you want to give access to a greater or lesser portion of vCPUs to a specific container, use Kubernetes cpu requests.

Kubernetes

# ~/cpu-demo.yaml
apiVersion: v1
kind: Pod
metadata:
  name: cpu-demo
  namespace: sandbox
spec:
  containers:
  - name: cpu0
    image: vish/stress
    resources:
      requests:
        cpu: "0.7"
    args:
    - -cpus
    - "3"
$ sudo -E kubectl create -f ~/cpu-demo.yaml

Before running containers without CPU constraint, consider that your containers are not running alone. Since your containers run inside a virtual machine other processes use the vCPUs as well (e.g. systemd and the Kata Containers agent). In general, we recommend setting default_vcpus equal to 1 to allow non-container processes to run on this vCPU and to specify a CPU constraint for each container.

Container with CPU constraint

The runtime calculates the number of vCPUs required by a container with CPU constraints using the following formula: vCPUs = ceiling( quota / period ), where quota specifies the number of microseconds per CPU Period that the container is guaranteed CPU access and period specifies the CPU CFS scheduler period of time in microseconds. The result determines the number of vCPU to hot plug into the virtual machine. Once the vCPUs have been added, the agent places the container inside a CPU cgroup. This placement allows the container to use only its assigned resources.

Do not waste resources

If you already know the number of vCPUs needed for each container and pod, or just want to run them with the same number of vCPUs, you can specify that number using the default_vcpus option in the configuration file, each virtual machine starts with that number of vCPUs. One limitation of this approach is that these vCPUs cannot be removed later and you might be wasting resources. For example, if you set default_vcpus to 8 and run only one container with a CPU constraint of 1 vCPUs, you might be wasting 7 vCPUs since the virtual machine starts with 8 vCPUs and 1 vCPUs is added and assigned to the container. Non-container processes might be able to use 8 vCPUs but they use a maximum 1 vCPU, hence 7 vCPUs might not be used.

Virtual CPU handling without hotplug

In some cases, the hardware and/or software architecture being utilized does not support hotplug. For example, Firecracker VMM does not support CPU or memory hotplug. Similarly, the current Linux Kernel for aarch64 does not support CPU or memory hotplug. To appropriately size the virtual machine for the workload within the container or pod, we provide a static_sandbox_resource_mgmt flag within the Kata Containers configuration. When this is set, the runtime will:

  • Size the VM based on the workload requirements as well as the default_vcpus option specified in the configuration.
  • Not resize the virtual machine after it has been launched.

VM size determination varies depending on the type of container being run, and may not always be available. If workload sizing information is not available, the virtual machine will be started with the default_vcpus.

In the case of a pod, the initial sandbox container (pause container) typically doesn't contain any resource information in its runtime spec. It is possible that the upper layer runtime (i.e. containerd or CRI-O) may pass sandbox sizing annotations within the pause container's spec. If these are provided, we will use this to appropriately size the VM. In particular, we'll calculate the number of CPUs required for the workload and augment this by default_vcpus configuration option, and use this for the virtual machine size.

In the case of a single container (i.e., not a pod), if the container specifies resource requirements, the container's spec will provide the sizing information directly. If these are set, we will calculate the number of CPUs required for the workload and augment this by default_vcpus configuration option, and use this for the virtual machine size.