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Node Allocatable resources proposal
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Tim St. Clair
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docs/proposals/node-allocatable.md
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docs/proposals/node-allocatable.md
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
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If you are using a released version of Kubernetes, you should
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refer to the docs that go with that version.
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<strong>
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The latest release of this document can be found
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[here](http://releases.k8s.io/release-1.1/docs/proposals/node-allocatable.md).
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Documentation for other releases can be found at
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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# Node Allocatable Resources
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**Issue:** https://github.com/kubernetes/kubernetes/issues/13984
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## Overview
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Currently Node.Status has Capacity, but no concept of node Allocatable. We need additional
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parameters to serve several purposes:
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1. [Kubernetes metrics](compute-resource-metrics-api.md) provides "/docker-daemon", "/kubelet",
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"/kube-proxy", "/system" etc. raw containers for monitoring system component resource usage
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patterns and detecting regressions. Eventually we want to cap system component usage to a certain
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limit / request. However this is not currently feasible due to a variety of reasons including:
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1. Docker still uses tons of computing resources (See
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[#16943](https://github.com/kubernetes/kubernetes/issues/16943))
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2. We have not yet defined the minimal system requirements, so we cannot control Kubernetes
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nodes or know about arbitrary daemons, which can make the system resources
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unmanageable. Even with a resource cap we cannot do a full resource management on the
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node, but with the proposed parameters we can mitigate really bad resource over commits
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3. Usage scales with the number of pods running on the node
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2. For external schedulers (such as mesos, hadoop, etc.) integration, they might want to partition
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compute resources on a given node, limiting how much Kubelet can use. We should provide a
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mechanism by which they can query kubelet, and reserve some resources for their own purpose.
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### Scope of proposal
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This proposal deals with resource reporting through the [`Allocatable` field](#allocatable) for more
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reliable scheduling, and minimizing resource over commitment. This proposal *does not* cover
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resource usage enforcement (e.g. limiting kubernetes component usage), pod eviction (e.g. when
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reservation grows), or running multiple Kubelets on a single node.
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## Design
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### Definitions
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1. **Node Capacity** - Already provided as
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[`NodeStatus.Capacity`](https://htmlpreview.github.io/?https://github.com/kubernetes/kubernetes/HEAD/docs/api-reference/v1/definitions.html#_v1_nodestatus),
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this is total capacity read from the node instance, and assumed to be constant.
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2. **System-Reserved** (proposed) - Compute resources reserved for processes which are not managed by
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Kubernetes. Currently this covers all the processes lumped together in the `/system` raw
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container.
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3. **Kubelet Allocatable** - Compute resources available for scheduling (including scheduled &
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unscheduled resources). This value is the focus of this proposal. See [below](#api-changes) for
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more details.
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4. **Kube-Reserved** (proposed) - Compute resources reserved for Kubernetes components such as the
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docker daemon, kubelet, kube proxy, etc.
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### API changes
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#### Allocatable
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Add `Allocatable` (4) to
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[`NodeStatus`](https://htmlpreview.github.io/?https://github.com/kubernetes/kubernetes/HEAD/docs/api-reference/v1/definitions.html#_v1_nodestatus):
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```
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type NodeStatus struct {
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...
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// Allocatable represents schedulable resources of a node.
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Allocatable ResourceList `json:"allocatable,omitempty"`
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...
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}
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```
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Allocatable will be computed by the Kubelet and reported to the API server. It is defined to be:
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```
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[Allocatable] = [Node Capacity] - [Kube-Reserved] - [System-Reserved]
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```
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The scheduler will use `Allocatable` in place of `Capacity` when scheduling pods, and the Kubelet
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will use it when performing admission checks.
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*Note: Since kernel usage can fluctuate and is out of kubernetes control, it will be reported as a
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separate value (probably via the metrics API). Reporting kernel usage is out-of-scope for this
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proposal.*
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#### Kube-Reserved
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`KubeReserved` is the parameter specifying resources reserved for kubernetes components (4). It is
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provided as a command-line flag to the Kubelet at startup, and therefore cannot be changed during
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normal Kubelet operation (this may change in the [future](#future-work)).
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The flag will be specified as a serialized `ResourceList`, with resources defined by the API
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`ResourceName` and values specified in `resource.Quantity` format, e.g.:
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```
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--kube-reserved=cpu=500m,memory=5Mi
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```
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Initially we will only support CPU and memory, but will eventually support more resources. See
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[#16889](https://github.com/kubernetes/kubernetes/pull/16889) for disk accounting.
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If KubeReserved is not set it defaults to a sane value (TBD) calculated from machine capacity. If it
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is explicitly set to 0 (along with `SystemReserved`), then `Allocatable == Capacity`, and the system
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behavior is equivalent to the 1.1 behavior with scheduling based on Capacity.
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#### System-Reserved
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In the initial implementation, `SystemReserved` will be functionally equivalent to
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[`KubeReserved`](#system-reserved), but with a different semantic meaning. While KubeReserved
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designates resources set asside for kubernetes components, SystemReserved designates resources set
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asside for non-kubernetes components (currently this is reported as all the processes lumped
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together in the `/system` raw container).
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## Issues
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### Kubernetes reservation is smaller than kubernetes component usage
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**Solution**: Initially, do nothing (best effort). Let the kubernetes daemons overflow the reserved
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resources and hope for the best. If the node usage is less than Allocatable, there will be some room
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for overflow and the node should continue to function. If the node has been scheduled to capacity
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(worst-case scenario) it may enter an unstable state, which is the current behavior in this
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situation.
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In the [future](#future-work) we may set a parent cgroup for kubernetes components, with limits set
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according to `KubeReserved`.
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### Version discrepancy
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**API server / scheduler is not allocatable-resources aware:** If the Kubelet rejects a Pod but the
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scheduler expects the Kubelet to accept it, the system could get stuck in an infinite loop
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scheduling a Pod onto the node only to have Kubelet repeatedly reject it. To avoid this situation,
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we will do a 2-stage rollout of `Allocatable`. In stage 1 (targeted for 1.2), `Allocatable` will
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be reported by the Kubelet and the scheduler will be updated to use it, but Kubelet will continue
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to do admission checks based on `Capacity` (same as today). In stage 2 of the rollout (targeted
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for 1.3 or later), the Kubelet will start doing admission checks based on `Allocatable`.
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**API server expects `Allocatable` but does not receive it:** If the kubelet is older and does not
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provide `Allocatable` in the `NodeStatus`, then `Allocatable` will be
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[defaulted](../../pkg/api/v1/defaults.go) to
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`Capacity` (which will yield todays behavior of scheduling based on capacity).
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### 3rd party schedulers
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The community should be notified that an update to schedulers is recommended, but if a scheduler is
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not updated it falls under the above case of "scheduler is not allocatable-resources aware".
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## Future work
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1. Convert kubelet flags to Config API - Prerequisite to (2). See
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[#12245](https://github.com/kubernetes/kubernetes/issues/12245).
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2. Set cgroup limits according KubeReserved - as described in the [overview](#overview)
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3. Report kernel usage to be considered with scheduling decisions.
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