5.7. Configuring higher VM workload density

Prerequisites

• You have installed the OpenShift CLI (oc).
• You are logged into the cluster with the cluster-admin role.
• A memory overcommit ratio is defined.
• The node belongs to a worker pool.

Procedure

• Provision swap by creating a MachineConfig object:

  1. Create a MachineConfig file with the parameters shown in the following example:

     apiVersion: machineconfiguration.openshift.io/v1
     kind: MachineConfig
     metadata:
       labels:
         machineconfiguration.openshift.io/role: worker
       name: 90-worker-swap
     spec:
       config:
         ignition:
           version: 3.5.0
         storage:
           files:
           - contents:
               source: data:text/plain;charset=utf-8;base64,YXBpVmVyc2lvbjoga3ViZWxldC5jb25maWcuazhzLmlvL3YxYmV0YTEKa2luZDogS3ViZWxldENvbmZpZ3VyYXRpb24KZmFpbFN3YXBPbjogZmFsc2UK
             mode: 420
             overwrite: true
             path: /etc/openshift/kubelet.conf.d/90-swap.conf
         systemd:
           units:
             - contents: |
                 [Unit]
                 Description=Enable swap
                 ConditionFirstBoot=no
                 ConditionPathExists=/var/tmp/swapfile
     
                 [Service]
                 Type=oneshot
                 ExecStart=/bin/sh -c "sudo swapon /var/tmp/swapfile"
     
                 [Install]
                 RequiredBy=kubelet-dependencies.target
               enabled: true
               name: swap-enable.service
             - contents: |
                 [Unit]
                 Description=Provision and enable swap
                 ConditionFirstBoot=no
                 ConditionPathExists=!/var/tmp/swapfile
     
                 [Service]
                 Type=oneshot
                 Environment=SWAP_SIZE_MB=5000
                 ExecStart=/bin/sh -c "sudo fallocate -l ${SWAP_SIZE_MB}M /var/tmp/swapfile && \
                 sudo chmod 600 /var/tmp/swapfile && \
                 sudo mkswap /var/tmp/swapfile && \
                 sudo swapon /var/tmp/swapfile && \
                 free -h"
     
                 [Install]
                 RequiredBy=kubelet-dependencies.target
               enabled: true
               name: swap-provision.service
             - contents: |
                 [Unit]
                 Description=Restrict swap for system slice
                 ConditionFirstBoot=no
     
                 [Service]
                 Type=oneshot
                 ExecStart=/bin/sh -c "sudo systemctl set-property --runtime system.slice MemorySwapMax=0 IODeviceLatencyTargetSec=\"/ 50ms\""
     
                 [Install]
                 RequiredBy=kubelet-dependencies.target
               enabled: true
               name: cgroup-system-slice-config.service

     To have enough swap space for the worst-case scenario, make sure to have at least as much swap space provisioned as overcommitted RAM. Calculate the amount of swap space to be provisioned on a node by using the following formula:

     NODE_SWAP_SPACE = NODE_RAM * (MEMORY_OVER_COMMIT_PERCENT / 100% - 1)

     Example:

     NODE_SWAP_SPACE = 16 GB * (150% / 100% - 1)
                    = 16 GB * (1.5 - 1)
                    = 16 GB * (0.5)
                    =  8 GB

  2. Wait for the worker nodes to sync with the new configuration by running the following command:

     $ oc wait mcp worker --for condition=Updated=True --timeout=-1s

     1. Enable memory overcommitment in OpenShift Virtualization by using the web console or the CLI.

        • Web console

          1. In the OpenShift Container Platform web console, go to Virtualization Overview Settings General settings Memory density.
          2. Set Configure memory density to on.
          3. Expand the Current memory density line.

          4. Set the density value by moving the Requested memory density slider. You can increase the density from 100% up to 400% in increments of 25%.

             The Memory density field shows the actual and requested values.

          5. Click Save.

        • CLI

          • Configure your OpenShift Virtualization to enable higher memory density and set the overcommit rate:

            $ oc -n openshift-cnv patch HyperConverged/kubevirt-hyperconverged --type='json' -p='[ \
              { \
              "op": "replace", \
              "path": "/spec/higherWorkloadDensity/memoryOvercommitPercentage", \
              "value": 150 \
              } \
            ]'

            Successful output:

            hyperconverged.hco.kubevirt.io/kubevirt-hyperconverged patched