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docs: Update vcpu handling document

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This PR updates the vcpu handling document by removing docker information
which is not longer being used in kata 2.x and leaving only k8s information.

Fixes #3950

Signed-off-by: Gabriela Cervantes <gabriela.cervantes.tellez@intel.com>
This commit is contained in:
Gabriela Cervantes 2022-03-23 17:58:49 +00:00
parent 7a8b96b857
commit 9a5b477062
1 changed files with 13 additions and 51 deletions
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64
docs/design/vcpu-handling.md
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@ -2,24 +2,15 @@
## Default number of virtual CPUs
Before starting a container, the [runtime][6] reads the `default_vcpus` option
from the [configuration file][7] to determine the number of virtual CPUs
Before starting a container, the [runtime][4] reads the `default_vcpus` option
from the [configuration file][5] 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
[docker `--cpus`][1], [docker update][4], or [Kubernetes `cpu` limits][2] to
assign more resources.
*Docker*
```sh
$ docker run --name foo -ti --cpus 2 debian bash
$ docker update --cpus 4 foo
```
[Kubernetes `cpu` limits][1] to assign more resources.
*Kubernetes*
@ -49,7 +40,7 @@ $ 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][3]].
network, and a specification for how to run the containers [[specification][2]].
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.
@ -73,13 +64,7 @@ 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 [`docker --cpu-shares`][1] or [Kubernetes `cpu` requests][2].
*Docker*
```sh
$ docker run -ti --cpus-shares=512 debian bash
```
use [Kubernetes `cpu` requests][1].
*Kubernetes*
@ -109,10 +94,9 @@ $ 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][5]). In general, we recommend setting `default_vcpus` equal to 1 to
[agent][3]). 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. If your container is already running and needs
more vCPUs, you can add more using [docker update][4].
constraint for each container.
## Container with CPU constraint
@ -121,7 +105,7 @@ constraints using the following formula: `vCPUs = ceiling( quota / period )`, wh
`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][5] places the
virtual machine. Once the vCPUs have been added, the [agent][3] places the
container inside a CPU cgroup. This placement allows the container to use only
its assigned resources.
@ -138,25 +122,6 @@ 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.
*Container without CPU constraint*
```sh
$ docker run -ti debian bash -c "nproc; cat /sys/fs/cgroup/cpu,cpuacct/cpu.cfs_*"
1 # number of vCPUs
100000 # cfs period
-1 # cfs quota
```
*Container with CPU constraint*
```sh
docker run --cpus 4 -ti debian bash -c "nproc; cat /sys/fs/cgroup/cpu,cpuacct/cpu.cfs_*"
5 # number of vCPUs
100000 # cfs period
400000 # cfs quota
```
## Virtual CPU handling without hotplug
In some cases, the hardware and/or software architecture being utilized does not support
@ -183,11 +148,8 @@ the container's `spec` will provide the sizing information directly. If these ar
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
[3]: https://kubernetes.io/docs/concepts/workloads/pods/pod/
[4]: https://docs.docker.com/engine/reference/commandline/update/
[5]: ../../src/agent
[6]: ../../src/runtime
[7]: ../../src/runtime/README.md#configuration
[1]: https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-resource
[2]: https://kubernetes.io/docs/concepts/workloads/pods/pod/
[3]: ../../src/agent
[4]: ../../src/runtime
[5]: ../../src/runtime/README.md#configuration