Adding CPUs on the fly

Imagine an enterprise having to correctly add dimension to all their systems right from the start. In my experience, this is very difficult. You will either underdimension it, and your customers will complain about performance at some point, or you will overdimension it, and then the machine will sit there, idling about, which is not optimal either. This is the reason hardware vendors have come up with hot-add resources. This allows a system to have its CPUs, memory, and/or disks to be upgraded/increased without the need for a shutdown. A KVM implements a similar functionality for its guests. It allows you to increase the CPUs, memory, and disks on the fly.

The actual recipe is very simple to execute, but there are some prerequisites to be met.

Getting ready

In order to be able to add CPUs on the fly to a guest, the guest's configuration must support them.

There are two ways to achieve this:

  • It must be created with the max option, as follows:
    --vcpus 2,maxvcpus=4
  • You can set the maximum using virsh (which will be applied at the next boot) through the following command:
    ~]# virsh setvcpus --domain <guestname> --count <max cpu count> --config --maximum
  • You can edit the guests' XML files, as follows:
    ~]# virsh edit <guestname>

The last two options will require you to shut down and boot (not reboot) your guest as these commands cannot change the "live" configuration.

The guest's XML file must contain the following element with the subsequent attributes:

<domain type='kvm'>
...
<vcpu current='2'>4</vcpu>
...
</domain>

Here, current indicates the number of CPUs in use, and the number within the node indicates the maximum number of vCPUs that can be assigned. This number can be increased but should never exceed the number of cores or threads in your host.

How to do it…

Let's add some CPUs to the guest.

On the KVM host, perform the following steps:

  1. Get the maximum number vCPUs that you can assign, as follows:
    ~]# virsh dumpxml <guestname> |grep vcpu
<vcpu placement='static' current='4'>8</vcpu>
  2. Now, set the new number of vCPUs through this command:
    ~]# virsh setvcpus --domai
n <guestname> --count <# of CPUs> --live

On the KVM guest, perform the following:

  1. Tell your guest OS there are more CPUs available by executing the following command:
    ~]# for i in $(grep -H 0 /sys/devices/system/cpu/cpu*/online | awk -F: '{print $1}'); do echo 1 > $i; done
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