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| Original file line number | Diff line number | Diff line change |
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| - [Host cgroup management](#host-cgroup-management) | ||
| - [Introduction](#introduction) | ||
| - [`SandboxCgroupOnly` enabled](#sandboxcgrouponly-enabled) | ||
| - [What does Kata do in this configuration?](#what-does-kata-do-in-this-configuration) | ||
| - [Why create a Kata-cgroup under the parent cgroup?](#why-create-a-kata-cgroup-under-the-parent-cgroup) | ||
| - [Improvements](#improvements) | ||
| - [`SandboxCgroupOnly` disabled (default, legacy)](#sandboxcgrouponly-disabled-default-legacy) | ||
| - [What does this method do?](#what-does-this-method-do) | ||
| - [Impact](#impact) | ||
| - [Summary](#summary) | ||
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| # Host cgroup management | ||
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| ## Introduction | ||
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| In Kata Containers, workloads run in a virtual machine that is managed by a virtual | ||
| machine monitor (VMM) running on the host. As a result, Kata Containers run over two layers of cgroups. The | ||
| first layer is in the guest where the workload is placed, while the second layer is on the host where the | ||
| VMM and associated threads are running. | ||
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| The OCI [runtime specification][linux-config] provides guidance on where the container cgroups should be placed: | ||
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| > [`cgroupsPath`][cgroupspath]: (string, OPTIONAL) path to the cgroups. It can be used to either control the cgroups | ||
| > hierarchy for containers or to run a new process in an existing container | ||
| cgroups are hierarchical, and this can be seen with the following pod example: | ||
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| - Pod 1: `cgroupsPath=/kubepods/pod1` | ||
| - Container 1: | ||
| `cgroupsPath=/kubepods/pod1/container1` | ||
| - Container 2: | ||
| `cgroupsPath=/kubepods/pod1/container2` | ||
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| - Pod 2: `cgroupsPath=/kubepods/pod2` | ||
| - Container 1: | ||
| `cgroupsPath=/kubepods/pod2/container2` | ||
| - Container 2: | ||
| `cgroupsPath=/kubepods/pod2/container2` | ||
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| Depending on the upper-level orchestrator, the cgroup under which the pod is placed is | ||
| managed by the orchestrator. In the case of Kubernetes, the pod-cgroup is created by Kubelet, | ||
| while the container cgroups are to be handled by the runtime. Kubelet will size the pod-cgroup | ||
| based on the container resource requirements. | ||
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| Kata Containers introduces a non-negligible overhead for running a sandbox (pod). Based on this, two scenarios are possible: | ||
| 1) The upper-layer orchestrator takes the overhead of running a sandbox into account when sizing the pod-cgroup, or | ||
| 2) Kata Containers do not fully constrain the VMM and associated processes, instead placing a subset of them outside of the pod-cgroup. | ||
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| Kata Containers provides two options for how cgroups are handled on the host. Selection of these options is done through | ||
| the `SandboxCgroupOnly` flag within the Kata Containers [configuration](https://github.com/kata-containers/runtime#configuration) | ||
| file. | ||
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| ## `SandboxCgroupOnly` enabled | ||
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| With `SandboxCgroupOnly` enabled, it is expected that the parent cgroup is sized to take the overhead of running | ||
| a sandbox into account. This is ideal, as all the applicable Kata Containers components can be placed within the | ||
| given cgroup-path. | ||
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| In the context of Kubernetes, Kubelet will size the pod-cgroup to take the overhead of running a Kata-based sandbox | ||
| into account. This will be feasible in the 1.16 Kubernetes release through the `PodOverhead` feature. | ||
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| ``` | ||
| +----------------------------------------------------------+ | ||
| | +---------------------------------------------------+ | | ||
| | | +---------------------------------------------+ | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | | kata-shimv2, VMM and threads: | | | | | ||
| | | | | (VMM, IO-threads, vCPU threads, etc)| | | | | ||
| | | | | | | | | | ||
| | | | | kata-sandbox-<id> | | | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | | | | | ||
| | | |Pod 1 | | | | ||
| | | +---------------------------------------------+ | | | ||
| | | | | | ||
| | | +---------------------------------------------+ | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | | kata-shimv2, VMM and threads: | | | | | ||
| | | | | (VMM, IO-threads, vCPU threads, etc)| | | | | ||
| | | | | | | | | | ||
| | | | | kata-sandbox-<id> | | | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | |Pod 2 | | | | ||
| | | +---------------------------------------------+ | | | ||
| | |kubepods | | | ||
| | +---------------------------------------------------+ | | ||
| | | | ||
| |Node | | ||
| +----------------------------------------------------------+ | ||
| ``` | ||
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| ### What does Kata do in this configuration? | ||
| 1. Given a `PodSandbox` container creation, let: | ||
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| ``` | ||
| podCgroup=Parent(container.CgroupsPath) | ||
| KataSandboxCgroup=<podCgroup>/kata-sandbox-<PodSandboxID> | ||
| ``` | ||
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| 2. Create the cgroup, `KataSandboxCgroup` | ||
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| 3. Join the `KataSandboxCgroup` | ||
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| Any process created by the runtime will be created in `KataSandboxCgroup`. | ||
| The runtime will not limit the cgroup in the host, but the caller is free | ||
| to set the proper limits for the `podCgroup`. | ||
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| In the example above the pod cgroups are `/kubepods/pod1` and `/kubepods/pod2`. | ||
| Kata creates the unrestricted sandbox cgroup under the pod cgroup. | ||
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| ### Why create a Kata-cgroup under the parent cgroup? | ||
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| `Docker` does not have a notion of pods, and will not create a cgroup directory | ||
| to place a particular container in (i.e., all containers would be in a path like | ||
| `/docker/container-id`. To simplify the implementation and continue to support `Docker`, | ||
| Kata Containers creates the sandbox-cgroup, in the case of Kubernetes, or a container cgroup, in the case | ||
| of docker. | ||
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| ### Improvements | ||
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| - Get statistics about pod resources | ||
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| If the Kata caller wants to know the resource usage on the host it can get | ||
| statistics from the pod cgroup. All cgroups stats in the hierarchy will include | ||
| the Kata overhead. This gives the possibility of gathering usage-statics at the | ||
| pod level and the container level. | ||
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| - Better host resource isolation | ||
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| Because the Kata runtime will place all the Kata processes in the pod cgroup, | ||
| the resource limits that the caller applies to the pod cgroup will affect all | ||
| processes that belong to the Kata sandbox in the host. This will improve the | ||
| isolation in the host preventing Kata to become a noisy neighbor. | ||
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| ## `SandboxCgroupOnly` disabled (default, legacy) | ||
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| If the cgroup provided to Kata is not sized appropriately, instability will be | ||
| introduced when fully constraining Kata components, and the user-workload will | ||
| see a subset of resources that were requested. Based on this, the default | ||
| handling for Kata Containers is to not fully constrain the VMM and Kata | ||
| components on the host. | ||
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| ``` | ||
| +----------------------------------------------------------+ | ||
| | +---------------------------------------------------+ | | ||
| | | +---------------------------------------------+ | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | |Container 1 |-|Container 2 | | | | | ||
| | | | | |-| | | | | | ||
| | | | | Shim+container1 |-| Shim+container2 | | | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | | | | | ||
| | | |Pod 1 | | | | ||
| | | +---------------------------------------------+ | | | ||
| | | | | | ||
| | | +---------------------------------------------+ | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | |Container 1 |-|Container 2 | | | | | ||
| | | | | |-| | | | | | ||
| | | | | Shim+container1 |-| Shim+container2 | | | | | ||
| | | | +--------------------------------------+ | | | | ||
| | | | | | | | ||
| | | |Pod 2 | | | | ||
| | | +---------------------------------------------+ | | | ||
| | |kubepods | | | ||
| | +---------------------------------------------------+ | | ||
| | +---------------------------------------------------+ | | ||
| | | Hypervisor | | | ||
| | |Kata | | | ||
| | +---------------------------------------------------+ | | ||
| | | | ||
| |Node | | ||
| +----------------------------------------------------------+ | ||
| ``` | ||
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| ### What does this method do? | ||
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| 1. Given a container creation let `containerCgroupHost=container.CgroupsPath` | ||
| 1. Rename `containerCgroupHost` path to add `kata_` | ||
| 1. Let `PodCgroupPath=PodSanboxContainerCgroup` where `PodSanboxContainerCgroup` is the cgroup of a container of type `PodSandbox` | ||
| 1. Limit the `PodCgroupPath` with the sum of all the container limits in the Sandbox | ||
| 1. Move only vCPU threads of hypervisor to `PodCgroupPath` | ||
| 1. Per each container, move its `kata-shim` to its own `containerCgroupHost` | ||
| 1. Move hypervisor and applicable threads to memory cgroup `/kata` | ||
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| _Note_: the Kata Containers runtime will not add all the hypervisor threads to | ||
| the cgroup path requested, only vCPUs. These threads are run unconstrained. | ||
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| This mitigates the risk of the VMM and other threads receiving an out of memory scenario (`OOM`). | ||
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| #### Impact | ||
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| If resources are reserved at a system level to account for the overheads of | ||
| running sandbox containers, this configuration can be utilized with adequate | ||
| stability. In this scenario, non-negligible amounts of CPU and memory will be | ||
| utilized unaccounted for on the host. | ||
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| [linux-config]: https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md | ||
| [cgroupspath]: https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#cgroups-path | ||
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| ## Summary | ||
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| | cgroup option | default? | status | pros | cons | ||
| |-|-|-|-|-| | ||
| | `SandboxCgroupOnly=false` | yes | legacy | Easiest to make Kata work | Unaccounted for memory and resource utilization | ||
| | `SandboxCgroupOnly=true` | no | recommended | Complete tracking of Kata memory and CPU utilization. In Kubernetes, the Kubelet can fully constrain Kata via the pod cgroup | Requires upper layer orchestrator which sizes sandbox cgroup appropriately | |
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