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docs: describe vCPU handling when hotplug is unavailable

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Describe the static_sandbox_resource_mgmt flag, and how this applies to
configurations that do not utilize hotplug.

Signed-off-by: Eric Ernst <eric_ernst@apple.com>
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Eric Ernst 2022-01-12 00:00:30 -08:00
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docs/design/vcpu-handling.md
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@ -157,6 +157,32 @@ docker run --cpus 4 -ti debian bash -c "nproc; cat /sys/fs/cgroup/cpu,cpuacct/cp
400000 # cfs quota
```
## 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.
[1]: https://docs.docker.com/config/containers/resource_constraints/#cpu
[2]: https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-resource