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第10章 Configuring Real-Time Compute

In some use-cases, you might need instances on your Compute nodes to adhere to low-latency policies and perform real-time processing. Real-time Compute nodes include a real-time capable kernel, specific virtualization modules, and optimized deployment parameters, to facilitate real-time processing requirements and minimize latency.

The process to enable Real-time Compute includes:

  • configuring the BIOS settings of the Compute nodes
  • building a real-time image with real-time kernel and Real-Time KVM (RT-KVM) kernel module
  • assigning the ComputeRealTime role to the Compute nodes

For a use-case example of Real-time Compute deployment for NFV workloads, see the Example: Configuring OVS-DPDK with ODL and VXLAN tunnelling section in the Network Functions Virtualization Planning and Configuration Guide.

10.1. Preparing Your Compute Nodes for Real-Time


Real-time Compute nodes are supported only with Red Hat Enterprise Linux version 7.5 or later.

Before you can deploy Real-time Compute in your overcloud, you must enable Red Hat Enterprise Linux Real-Time KVM (RT-KVM), configure your BIOS to support real-time, and build the real-time image.


  • You must use Red Hat certified servers for your RT-KVM Compute nodes. See Red Hat Enterprise Linux for Real Time 7 certified servers for details.
  • You must enable the rhel-7-server-nfv-rpms repository for RT-KVM to build the real-time image.


    You need a separate subscription to Red Hat OpenStack Platform for Real Time before you can access this repository. For details on managing repositories and subscriptions for your undercloud, see the Registering and updating your undercloud section in the Director Installation and Usage guide.

    To check which packages will be installed from the repository, run the following command:

    $ yum repo-pkgs rhel-7-server-nfv-rpms list
    Loaded plugins: product-id, search-disabled-repos, subscription-manager
    Available Packages
    kernel-rt.x86_64                                                                     3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-debug.x86_64                                                               3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-debug-devel.x86_64                                                         3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-debug-kvm.x86_64                                                           3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-devel.x86_64                                                               3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-doc.noarch                                                                 3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    kernel-rt-kvm.x86_64                                                                 3.10.0-693.21.1.rt56.639.el7                                                       rhel-7-server-nfv-rpms
    [ output omitted…]

Building the real-time image

To build the overcloud image for Real-time Compute nodes:

  1. Install the libguestfs-tools package on the undercloud to get the virt-customize tool:

    (undercloud) [stack@undercloud-0 ~]$ sudo yum install libguestfs-tools

    If you install the libguestfs-tools package on the undercloud, disable iscsid.socket to avoid port conflicts with the tripleo_iscsid service on the undercloud:

    $ sudo systemctl disable --now iscsid.socket
  2. Extract the images:

    (undercloud) [stack@undercloud-0 ~]$ tar -xf /usr/share/rhosp-director-images/overcloud-full.tar
    (undercloud) [stack@undercloud-0 ~]$ tar -xf /usr/share/rhosp-director-images/ironic-python-agent.tar
  3. Copy the default image:

    (undercloud) [stack@undercloud-0 ~]$ cp overcloud-full.qcow2 overcloud-realtime-compute.qcow2
  4. Register the image and configure the required subscriptions:

    (undercloud) [stack@undercloud-0 ~]$  virt-customize -a overcloud-realtime-compute.qcow2 --run-command 'subscription-manager register --username=[username] --password=[password]'
    [  0.0] Examining the guest ...
    [ 10.0] Setting a random seed
    [ 10.0] Running: subscription-manager register --username=[username] --password=[password]
    [ 24.0] Finishing off

    Replace the username and password values with your Red Hat customer account details. For general information about building a Real-time overcloud image, see the Modifying the Red Hat Enterprise Linux OpenStack Platform Overcloud Image with virt-customize knowledgebase article.

  5. Find the SKU of the Red Hat OpenStack Platform for Real Time subscription. The SKU might be located on a system that is already registered to the Red Hat Subscription Manager with the same account and credentials. For example:

    $ sudo subscription-manager list
  6. Attach the Red Hat OpenStack Platform for Real Time subscription to the image:

    (undercloud) [stack@undercloud-0 ~]$  virt-customize -a overcloud-realtime-compute.qcow2 --run-command 'subscription-manager attach --pool [subscription-pool]'
  7. Create a script to configure rt on the image:

    (undercloud) [stack@undercloud-0 ~]$ cat
      set -eux
      subscription-manager repos --enable=rhel-7-server-rpms --enable=rhel-7-server-openstack-13-rpms --enable=rhel-7-server-nfv-rpms
      yum -v -y --setopt=protected_packages= erase kernel.$(uname -m)
      yum -v -y install kernel-rt kernel-rt-kvm tuned-profiles-nfv-host
  8. Run the script to configure the real-time image:

    (undercloud) [stack@undercloud-0 ~]$ virt-customize -a overcloud-realtime-compute.qcow2 -v --run 2>&1 | tee virt-customize.log
  9. Re-label SELinux:

    (undercloud) [stack@undercloud-0 ~]$ virt-customize -a overcloud-realtime-compute.qcow2 --selinux-relabel
  10. Extract vmlinuz and initrd:

    (undercloud) [stack@undercloud-0 ~]$ mkdir image
    (undercloud) [stack@undercloud-0 ~]$ guestmount -a overcloud-realtime-compute.qcow2 -i --ro image
    (undercloud) [stack@undercloud-0 ~]$ cp image/boot/vmlinuz-3.10.0-862.rt56.804.el7.x86_64 ./overcloud-realtime-compute.vmlinuz
    (undercloud) [stack@undercloud-0 ~]$ cp image/boot/initramfs-3.10.0-862.rt56.804.el7.x86_64.img ./overcloud-realtime-compute.initrd
    (undercloud) [stack@undercloud-0 ~]$ guestunmount image

    The software version in the vmlinuz and initramfs filenames vary with the kernel version.

  11. Upload the image:

    (undercloud) [stack@undercloud-0 ~]$ openstack overcloud image upload --update-existing --os-image-name overcloud-realtime-compute.qcow2

You now have a real-time image you can use with the ComputeRealTime composable role on select Compute nodes.

Modifying BIOS settings on Real-time Compute nodes

To reduce latency on your Real-time Compute nodes, you must modify the BIOS settings in the Compute nodes. You should disable all options for the following components in your Compute node BIOS settings:

  • Power Management
  • Hyper-Threading
  • CPU sleep states
  • Logical processors

See Setting BIOS parameters for descriptions of these settings and the impact of disabling them. See your hardware manufacturer documentation for complete details on how to change BIOS settings.