nodes-cluster-overcommit-node-memory.adoc

Reserving memory across quality of service tiers

You can use the qos-reserved parameter to specify a percentage of memory to be reserved by a pod in a particular QoS level. This feature attempts to reserve requested resources to exclude pods from lower OoS classes from using resources requested by pods in higher QoS classes.

By reserving resources for higher QOS levels, pods that do not have resource limits are prevented from encroaching on the resources requested by pods at higher QoS levels.

Prerequisites

1. Obtain the label associated with the static Machine Config Pool CRD for the type of node you want to configure. Perform one of the following steps:

   1. View the Machine Config Pool:

      $ oc describe machineconfigpool <name>

      For example:

      $ oc describe machineconfigpool worker
      
      apiVersion: machineconfiguration.openshift.io/v1
      kind: MachineConfigPool
      metadata:
        creationTimestamp: 2019-02-08T14:52:39Z
        generation: 1
        labels:
          custom-kubelet: small-pods (1)

      1. If a label has been added it appears under labels.

   2. If the label is not present, add a key/value pair:

      $ oc label machineconfigpool worker custom-kubelet=small-pods

      Tip	You can alternatively apply the following YAML to add the label: apiVersion: machineconfiguration.openshift.io/v1 kind: MachineConfigPool metadata: labels: custom-kubelet: small-pods name: worker

Procedure

1. Create a Custom Resource (CR) for your configuration change.

   Sample configuration for a disabling CPU limits

   apiVersion: machineconfiguration.openshift.io/v1
   kind: KubeletConfig
   metadata:
     name: disable-cpu-units (1)
   spec:
     machineConfigPoolSelector:
       matchLabels:
         custom-kubelet: small-pods (2)
     kubeletConfig:
       cgroups-per-qos:
       - true
       cgroup-driver:
       - 'systemd'
       cgroup-root:
       - '/'
       qos-reserved: (3)
       - 'memory=50%'

   1. Assign a name to CR.

   2. Specify the label to apply the configuration change.

   3. Specifies how pod resource requests are reserved at the QoS level. {product-title} uses the qos-reserved parameter as follows:

      • A value of qos-reserved=memory=100% will prevent the Burstable and BestEffort QOS classes from consuming memory that was requested by a higher QoS class. This increases the risk of inducing OOM on BestEffort and Burstable workloads in favor of increasing memory resource guarantees for Guaranteed and Burstable workloads.

      • A value of qos-reserved=memory=50% will allow the Burstable and BestEffort QOS classes to consume half of the memory requested by a higher QoS class.

      • A value of qos-reserved=memory=0% will allow a Burstable and BestEffort QoS classes to consume up to the full node allocatable amount if available, but increases the risk that a Guaranteed workload will not have access to requested memory. This condition effectively disables this feature.