| 1 | <?xml version='1.0' encoding='UTF-8'?>
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| 2 | <!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
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| 3 | <topic xml:lang="en-us" id="cpuhotplug">
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| 4 | <title>CPU Hot-Plugging</title>
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| 5 |
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| 6 | <body>
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| 7 | <p>
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| 8 | With virtual machines running modern server operating systems,
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| 9 | Oracle VM VirtualBox supports CPU hot-plugging.
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| 10 | </p>
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| 11 | <p>
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| 12 | On a physical computer CPU hot-plugging would mean that a CPU can
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| 13 | be added or removed while the machine is running. Oracle VM VirtualBox
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| 14 | supports adding and removing of virtual CPUs while a virtual
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| 15 | machine is running.
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| 16 | </p>
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| 17 | <p>
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| 18 | CPU hot-plugging works only with guest operating systems that
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| 19 | support the feature. So far this applies only to Linux and Windows
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| 20 | Server. Windows supports only hot-add, while Linux supports
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| 21 | hot-add and hot-remove. To use this feature with more than 8 CPUs,
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| 22 | a 64-bit Linux guest is required.
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| 23 | </p>
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| 24 | <p>
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| 25 | CPU hot-plugging is done using the <userinput>VBoxManage</userinput>
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| 26 | command-line interface. First, hot-plugging needs to be enabled
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| 27 | for a virtual machine:
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| 28 | </p>
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| 29 | <pre xml:space="preserve">$ VBoxManage modifyvm <varname>VM-name</varname> --cpu-hotplug on</pre>
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| 30 | <p>
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| 31 | The <codeph>--cpus</codeph> option is used to specify the maximum
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| 32 | number of CPUs that the virtual machine can have:
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| 33 | </p>
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| 34 | <pre xml:space="preserve">$ VBoxManage modifyvm <varname>VM-name</varname> --cpus 8</pre>
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| 35 | <p>
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| 36 | When the VM is off, you can then add and remove virtual CPUs with
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| 37 | the <userinput>VBoxManage modifyvm --plug-cpu</userinput> and
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| 38 | <userinput>VBoxManage modifyvm --unplug-cpu</userinput> commands,
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| 39 | which take the number of the virtual CPU as a parameter, as
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| 40 | follows:
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| 41 | </p>
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| 42 | <pre xml:space="preserve">$ VBoxManage modifyvm <varname>VM-name</varname> --plug-cpu 3
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| 43 | $ VBoxManage modifyvm <varname>VM-name</varname> --unplug-cpu 3</pre>
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| 44 | <p>
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| 45 | Note that CPU 0 can never be removed.
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| 46 | </p>
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| 47 | <p>
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| 48 | While the VM is running, CPUs can be added and removed with the
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| 49 | <userinput>VBoxManage controlvm plugcpu</userinput> and
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| 50 | <userinput>VBoxManage controlvm unplugcpu</userinput> commands
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| 51 | instead, as follows:
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| 52 | </p>
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| 53 | <pre xml:space="preserve">$ VBoxManage controlvm <varname>VM-name</varname> plugcpu 3
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| 54 | $ VBoxManage controlvm <varname>VM-name</varname> unplugcpu 3</pre>
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| 55 | <p>
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| 56 | See <xref href="vboxmanage-modifyvm.dita">VBoxManage modifyvm</xref> and
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| 57 | <xref href="vboxmanage-controlvm.dita">VBoxManage controlvm</xref> for details.
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| 58 | </p>
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| 59 | <p>
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| 60 | With Linux guests, the following applies:
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| 61 | </p>
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| 62 | <p>
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| 63 | To prevent ejection while the CPU is still used it has to be
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| 64 | ejected from within the guest before. The Linux Guest Additions
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| 65 | contain a service which receives hot-remove events and ejects the
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| 66 | CPU. Also, after a CPU is added to the VM it is not automatically
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| 67 | used by Linux. The Linux Guest Additions service will take care of
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| 68 | that if installed. If not a CPU can be started with the following
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| 69 | command:
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| 70 | </p>
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| 71 | <pre xml:space="preserve">$ echo 1 > /sys/devices/system/cpu/cpu<id>/online</pre>
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| 72 | </body>
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| 73 |
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| 74 | </topic>
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