Performance Optimization

After the initial creation of your Virtual Machine there are a number of performance tweaks you can make to your Guest's .XML file and/or the Host system to greatly increase the Guest's performance.

hugepages

Hugepages are a function that lets the system kernel use larger pages when reading or writing information to memory (RAM). When this is enabled and the Guest is configured to use them the performance can be greatly increased. How to enable Hugepages depends on your GNU/Linux distribution.

Note: When hugepages are configured this portion of memory is taken away from the Host. This means the Host will no longer be able to use it. Keep this in mind.

Debain (Ubuntu/Mint/Lubuntu/PopOS/etc)

First check if Linux isn't already using Hugepages with: cat /proc/meminfo | grep Huge. If the output resembles the following:

AnonHugePages:       2048 kB
ShmemHugePages:         0 kB
HugePages_Total:        0
HugePages_Free:         0
HugePages_Rsvd:         0
HugePages_Surp:         0
Hugepagesize:        2048 kB
Hugetlb:                0 kB

then Hugepages aren't enabled.

To enable Hugepages first check /etc/sysctl.conf for the following entries:

vm.nr_hugepages=
vm.hugetlb_shm_group=

If they don't exist they can be appended to the end of the file.

The general rule of thumb is 1 Hugepage for every 2MB of of RAM to be assigned to the VM:

vm.nr_hugepages=8192
vm.hugetlb_shm_group=48

The above example will set aside approx 16GB. After saving the changes reboot the system.

Now to verify the changes rerun: cat /proc/meminfo | grep Huge The output should resemble the following:

AnonHugePages:          0 kB
ShmemHugePages:         0 kB
HugePages_Total:     8192
HugePages_Free:      8192
HugePages_Rsvd:         0
HugePages_Surp:         0
Hugepagesize:        2048 kB
Hugetlb:         16777216 kB

Hugepages are now enabled.

It's also recommended by RedHat to disable the old transparent hugepages. This can be done with:

echo 'never' > /sys/kernel/mm/transparent_hugepage/defrag
echo 'never' > /sys/kernel/mm/transparent_hugepage/enabled

Now restart the computer.

To make the VM use Hugepages enter the VM's .XML file and add <memoryBacking><hugepages/></memoryBacking> to the memory section:

...
<memory unit='KiB'>16777216</memory>
  <currentMemory unit='KiB'>16777216</currentMemory>
  <memoryBacking>
    <hugepages/>
  </memoryBacking>
...

Hugepages are now enabled.

hyperv

Hyperv has a number of variables that can change the way in which the VM interacts with system resources. A few variables that can help preserve resources for the system are by appending:

<vpindex state='on'/>
<runtime state='on'/>
<synic state='on'/>
<stimer state='on'/>

to the hyperv section of the VM's .XML file.

<hyperv>
  <related state='on'/>
  <vapic state='on'/>
  <spinlocks state='on' retries='8191'/>
  <vpindex state='on'/>
  <runtime state='on'/>
  <synic state='on'/>
  <stimer state='on'/>