Chapter 15. Improving cluster stability in high latency environments using worker latency profiles
If the cluster administrator has performed latency tests for platform verification, they can discover the need to adjust the operation of the cluster to ensure stability in cases of high latency. The cluster administrator need change only one parameter, recorded in a file, which controls four parameters affecting how supervisory processes read status and interpret the health of the cluster. Changing only the one parameter provides cluster tuning in an easy, supportable manner.
The Kubelet
process provides the starting point for monitoring cluster health. The Kubelet
sets status values for all nodes in the OpenShift Container Platform cluster. The Kubernetes Controller Manager (kube controller
) reads the status values every 10 seconds, by default. If the kube controller
cannot read a node status value, it loses contact with that node after a configured period. The default behavior is:
-
The node controller on the control plane updates the node health to
Unhealthy
and marks the nodeReady
condition`Unknown`. - In response, the scheduler stops scheduling pods to that node.
-
The Node Lifecycle Controller adds a
node.kubernetes.io/unreachable
taint with aNoExecute
effect to the node and schedules any pods on the node for eviction after five minutes, by default.
This behavior can cause problems if your network is prone to latency issues, especially if you have nodes at the network edge. In some cases, the Kubernetes Controller Manager might not receive an update from a healthy node due to network latency. The Kubelet
evicts pods from the node even though the node is healthy.
To avoid this problem, you can use worker latency profiles to adjust the frequency that the Kubelet
and the Kubernetes Controller Manager wait for status updates before taking action. These adjustments help to ensure that your cluster runs properly if network latency between the control plane and the worker nodes is not optimal.
These worker latency profiles contain three sets of parameters that are pre-defined with carefully tuned values to control the reaction of the cluster to increased latency. No need to experimentally find the best values manually.
You can configure worker latency profiles when installing a cluster or at any time you notice increased latency in your cluster network.
15.1. Understanding worker latency profiles
Worker latency profiles are four different categories of carefully-tuned parameters. The four parameters which implement these values are node-status-update-frequency
, node-monitor-grace-period
, default-not-ready-toleration-seconds
and default-unreachable-toleration-seconds
. These parameters can use values which allow you control the reaction of the cluster to latency issues without needing to determine the best values using manual methods.
Setting these parameters manually is not supported. Incorrect parameter settings adversely affect cluster stability.
All worker latency profiles configure the following parameters:
- node-status-update-frequency
- Specifies how often the kubelet posts node status to the API server.
- node-monitor-grace-period
-
Specifies the amount of time in seconds that the Kubernetes Controller Manager waits for an update from a kubelet before marking the node unhealthy and adding the
node.kubernetes.io/not-ready
ornode.kubernetes.io/unreachable
taint to the node. - default-not-ready-toleration-seconds
- Specifies the amount of time in seconds after marking a node unhealthy that the Kube API Server Operator waits before evicting pods from that node.
- default-unreachable-toleration-seconds
- Specifies the amount of time in seconds after marking a node unreachable that the Kube API Server Operator waits before evicting pods from that node.
The following Operators monitor the changes to the worker latency profiles and respond accordingly:
-
The Machine Config Operator (MCO) updates the
node-status-update-frequency
parameter on the worker nodes. -
The Kubernetes Controller Manager updates the
node-monitor-grace-period
parameter on the control plane nodes. -
The Kubernetes API Server Operator updates the
default-not-ready-toleration-seconds
anddefault-unreachable-toleration-seconds
parameters on the control plane nodes.
Although the default configuration works in most cases, OpenShift Container Platform offers two other worker latency profiles for situations where the network is experiencing higher latency than usual. The three worker latency profiles are described in the following sections:
- Default worker latency profile
With the
Default
profile, eachKubelet
updates its status every 10 seconds (node-status-update-frequency
). TheKube Controller Manager
checks the statuses ofKubelet
every 5 seconds (node-monitor-grace-period
).The Kubernetes Controller Manager waits 40 seconds for a status update from
Kubelet
before considering theKubelet
unhealthy. If no status is made available to the Kubernetes Controller Manager, it then marks the node with thenode.kubernetes.io/not-ready
ornode.kubernetes.io/unreachable
taint and evicts the pods on that node.If a pod on that node has the
NoExecute
taint, the pod is run according totolerationSeconds
. If the pod has no taint, it will be evicted in 300 seconds (default-not-ready-toleration-seconds
anddefault-unreachable-toleration-seconds
settings of theKube API Server
).Profile Component Parameter Value Default
kubelet
node-status-update-frequency
10s
Kubelet Controller Manager
node-monitor-grace-period
40s
Kubernetes API Server Operator
default-not-ready-toleration-seconds
300s
Kubernetes API Server Operator
default-unreachable-toleration-seconds
300s
- Medium worker latency profile
Use the
MediumUpdateAverageReaction
profile if the network latency is slightly higher than usual.The
MediumUpdateAverageReaction
profile reduces the frequency of kubelet updates to 20 seconds and changes the period that the Kubernetes Controller Manager waits for those updates to 2 minutes. The pod eviction period for a pod on that node is reduced to 60 seconds. If the pod has thetolerationSeconds
parameter, the eviction waits for the period specified by that parameter.The Kubernetes Controller Manager waits for 2 minutes to consider a node unhealthy. In another minute, the eviction process starts.
Profile Component Parameter Value MediumUpdateAverageReaction
kubelet
node-status-update-frequency
20s
Kubelet Controller Manager
node-monitor-grace-period
2m
Kubernetes API Server Operator
default-not-ready-toleration-seconds
60s
Kubernetes API Server Operator
default-unreachable-toleration-seconds
60s
- Low worker latency profile
Use the
LowUpdateSlowReaction
profile if the network latency is extremely high.The
LowUpdateSlowReaction
profile reduces the frequency of kubelet updates to 1 minute and changes the period that the Kubernetes Controller Manager waits for those updates to 5 minutes. The pod eviction period for a pod on that node is reduced to 60 seconds. If the pod has thetolerationSeconds
parameter, the eviction waits for the period specified by that parameter.The Kubernetes Controller Manager waits for 5 minutes to consider a node unhealthy. In another minute, the eviction process starts.
Profile Component Parameter Value LowUpdateSlowReaction
kubelet
node-status-update-frequency
1m
Kubelet Controller Manager
node-monitor-grace-period
5m
Kubernetes API Server Operator
default-not-ready-toleration-seconds
60s
Kubernetes API Server Operator
default-unreachable-toleration-seconds
60s
15.2. Implementing worker latency profiles at cluster creation
To edit the configuration of the installer, you will first need to use the command openshift-install create manifests
to create the default node manifest as well as other manifest YAML files. This file structure must exist before we can add workerLatencyProfile. The platform on which you are installing may have varying requirements. Refer to the Installing section of the documentation for your specific platform.
The workerLatencyProfile
must be added to the manifest in the following sequence:
- Create the manifest needed to build the cluster, using a folder name appropriate for your installation.
-
Create a YAML file to define
config.node
. The file must be in themanifests
directory. -
When defining
workerLatencyProfile
in the manifest for the first time, specify any of the profiles at cluster creation time:Default
,MediumUpdateAverageReaction
orLowUpdateSlowReaction
.
Verification
Here is an example manifest creation showing the
spec.workerLatencyProfile
Default
value in the manifest file:$ openshift-install create manifests --dir=<cluster-install-dir>
Edit the manifest and add the value. In this example we use
vi
to show an example manifest file with the "Default"workerLatencyProfile
value added:$ vi <cluster-install-dir>/manifests/config-node-default-profile.yaml
Example output
apiVersion: config.openshift.io/v1 kind: Node metadata: name: cluster spec: workerLatencyProfile: "Default"
15.3. Using and changing worker latency profiles
To change a worker latency profile to deal with network latency, edit the node.config
object to add the name of the profile. You can change the profile at any time as latency increases or decreases.
You must move one worker latency profile at a time. For example, you cannot move directly from the Default
profile to the LowUpdateSlowReaction
worker latency profile. You must move from the Default
worker latency profile to the MediumUpdateAverageReaction
profile first, then to LowUpdateSlowReaction
. Similarly, when returning to the Default
profile, you must move from the low profile to the medium profile first, then to Default
.
You can also configure worker latency profiles upon installing an OpenShift Container Platform cluster.
Procedure
To move from the default worker latency profile:
Move to the medium worker latency profile:
Edit the
node.config
object:$ oc edit nodes.config/cluster
Add
spec.workerLatencyProfile: MediumUpdateAverageReaction
:Example
node.config
objectapiVersion: config.openshift.io/v1 kind: Node metadata: annotations: include.release.openshift.io/ibm-cloud-managed: "true" include.release.openshift.io/self-managed-high-availability: "true" include.release.openshift.io/single-node-developer: "true" release.openshift.io/create-only: "true" creationTimestamp: "2022-07-08T16:02:51Z" generation: 1 name: cluster ownerReferences: - apiVersion: config.openshift.io/v1 kind: ClusterVersion name: version uid: 36282574-bf9f-409e-a6cd-3032939293eb resourceVersion: "1865" uid: 0c0f7a4c-4307-4187-b591-6155695ac85b spec: workerLatencyProfile: MediumUpdateAverageReaction 1 # ...
- 1
- Specifies the medium worker latency policy.
Scheduling on each worker node is disabled as the change is being applied.
Optional: Move to the low worker latency profile:
Edit the
node.config
object:$ oc edit nodes.config/cluster
Change the
spec.workerLatencyProfile
value toLowUpdateSlowReaction
:Example
node.config
objectapiVersion: config.openshift.io/v1 kind: Node metadata: annotations: include.release.openshift.io/ibm-cloud-managed: "true" include.release.openshift.io/self-managed-high-availability: "true" include.release.openshift.io/single-node-developer: "true" release.openshift.io/create-only: "true" creationTimestamp: "2022-07-08T16:02:51Z" generation: 1 name: cluster ownerReferences: - apiVersion: config.openshift.io/v1 kind: ClusterVersion name: version uid: 36282574-bf9f-409e-a6cd-3032939293eb resourceVersion: "1865" uid: 0c0f7a4c-4307-4187-b591-6155695ac85b spec: workerLatencyProfile: LowUpdateSlowReaction 1 # ...
- 1
- Specifies use of the low worker latency policy.
Scheduling on each worker node is disabled as the change is being applied.
Verification
When all nodes return to the
Ready
condition, you can use the following command to look in the Kubernetes Controller Manager to ensure it was applied:$ oc get KubeControllerManager -o yaml | grep -i workerlatency -A 5 -B 5
Example output
# ... - lastTransitionTime: "2022-07-11T19:47:10Z" reason: ProfileUpdated status: "False" type: WorkerLatencyProfileProgressing - lastTransitionTime: "2022-07-11T19:47:10Z" 1 message: all static pod revision(s) have updated latency profile reason: ProfileUpdated status: "True" type: WorkerLatencyProfileComplete - lastTransitionTime: "2022-07-11T19:20:11Z" reason: AsExpected status: "False" type: WorkerLatencyProfileDegraded - lastTransitionTime: "2022-07-11T19:20:36Z" status: "False" # ...
- 1
- Specifies that the profile is applied and active.
To change the medium profile to default or change the default to medium, edit the node.config
object and set the spec.workerLatencyProfile
parameter to the appropriate value.
15.4. Example steps for displaying resulting values of workerLatencyProfile
You can display the values in the workerLatencyProfile
with the following commands.
Verification
Check the
default-not-ready-toleration-seconds
anddefault-unreachable-toleration-seconds
fields output by the Kube API Server:$ oc get KubeAPIServer -o yaml | grep -A 1 default-
Example output
default-not-ready-toleration-seconds: - "300" default-unreachable-toleration-seconds: - "300"
Check the values of the
node-monitor-grace-period
field from the Kube Controller Manager:$ oc get KubeControllerManager -o yaml | grep -A 1 node-monitor
Example output
node-monitor-grace-period: - 40s
Check the
nodeStatusUpdateFrequency
value from the Kubelet. Set the directory/host
as the root directory within the debug shell. By changing the root directory to/host
, you can run binaries contained in the host’s executable paths:$ oc debug node/<worker-node-name> $ chroot /host # cat /etc/kubernetes/kubelet.conf|grep nodeStatusUpdateFrequency
Example output
“nodeStatusUpdateFrequency”: “10s”
These outputs validate the set of timing variables for the Worker Latency Profile.