Managing Virtual Machine Memory Parameters

This section describes how to configure memory parameters available for virtual machines:

Memory size
Video memory size
Memory hotplugging
Memory guarantees

Configuring Virtual Machine Memory Size

To increase or reduce the amount of memory that will be available to the virtual machine, use the --memsize option of the prlctl set command. The following example shows how to increase the RAM of the virtual machine MyVM from 1GB to 2GB and check that the new value has been successfully set:

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# prlctl list -i MyVM | grep memory
memory 1024Mb
# prlctl set MyVM --memsize 2048
Set the memsize parameter to 2048Mb
The VM has been successfully configured.
# prlctl list -i MyVM | grep memory
memory 2048Mb

The changes are saved in the VM configuration file and applied to the VM on start. If the VM is running, it will need to be rebooted. To be able to increase or reduce virtual machine RAM size without reboot, enable memory hotplugging as described in Enabling Virtual Machine Memory Hotplugging.

The value set with prlctl --memsize is not reported inside the VM as physical or other RAM size. A user logged in to the guest OS will see as much physical RAM as can be obtained by fully deflating the balloon (see MaxNumaSize in Enabling Virtual Machine Memory Hotplugging). The balloon size is not reported inside the VM as well. However, if the balloon is not fully deflated, a part of the reported physical RAM will appear to be occupied at all times (by what is in fact the balloon).

Configuring Virtual Machine Video Memory Size

To set the amount of video memory to be available to the virtual machine’s video card, use the --videosize option of the prlctl set command. Assuming that the current video memory size of the virtual machine MyVM is set to 32 MB, you can increase it to 64 MB by running the following command:

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# prlctl set MyVM --videosize 64

To check that the new value has been successfully set, use this command:

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# prlctl list -i MyVM | grep video
video 64Mb

Enabling Virtual Machine Memory Hotplugging

Memory hotplugging allows increasing or reducing virtual machine RAM size on the fly, without the need to reboot the VM. Memory hotplugging is implemented as a combination of ballooning and addition/removal of virtual DIMM slots.

The algorithm is as follows. When a command to increase VM memory size to RAM_size is run (as described in Configuring Virtual Machine Memory Size), the memory is first expanded by deflating the VM’s balloon. The balloon deflation limit, MaxNumaSize, is calculated automatically according to the formula

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MaxNumaSize = (RAM_size + 4GB) rounded up to a multiple of 4GB

If fully deflating the balloon is not enough to obtain RAM_size (that is, RAM_size exceeds MaxNumaSize), then memory is further expanded by adding virtual DIMM slots (up to twice the MaxNumaSize) and MaxNumaSize is set equal to RAM_size (that is, the maximum balloon size grows as well). When a command to decrease VM memory size is run, the memory is shrunk by inflating the VM’s balloon. The added virtual DIMM slots remain until VM restart. After restart, the VM has memory equal to RAM_size.

This feature is only supported for virtual machines with at least 1GB of RAM and is disabled by default. To enable it for a virtual machine (e.g., MyVM):

Make sure the VM is stopped.
Enable memory hotplugging for the VM:
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# prlctl set MyVM --mem-hotplug on
Start the VM.

Now virtual machine RAM size can be increased and decreased with the prlctl set --memsize command without rebooting the VM.

Configuring Virtual Machine Memory Guarantees

A memory guarantee is a percentage of virtual machine’s RAM that the VM is guaranteed to have.

The total memory guaranteed to all running virtual environments on the host must not exceed host’s physical RAM size. If starting a virtual environment with a memory guarantee would increase the total memory guarantee on the host beyond host’s physical RAM size, the virtual environment will not start. If setting a memory guarantee for a running virtual environment would increase the total memory guarantee on the host beyond host’s physical RAM size, the memory guarantee will not be set.

For virtual machines, the memory guarantee value is set to 80% by default. To change the default value, use the prlctl set --memguarantee command. For example:

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# prlctl set MyVM --memguarantee 60

To revert to the default setting, run

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# prlctl set MyVM --memguarantee auto

Virtual machines with memory guarantees can only be started with prlctl start. Starting such VMs differently (e.g., using virsh) will result in memory guarantees not being applied.