What influences virtual machine performance

Virtual machines (VMs) run as user-space processes on the host. The hypervisor needs to convert the host’s system resources so that the VMs can use them. A portion of the resources is consumed by the conversion, and the VM therefore cannot achieve the same performance efficiency as the host.

The impact of virtualization on system performance
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More specific reasons for VM performance loss include:

  • Virtual CPUs (vCPUs) are implemented as threads on the host, handled by the Linux scheduler.
  • VMs do not automatically inherit optimization features, such as NUMA or huge pages, from the host kernel.
  • Disk and network I/O settings of the host might have a significant performance impact on the VM.
  • Network traffic typically travels to a VM through a software-based bridge.
  • Depending on the host devices and their models, there might be significant overhead due to emulation of particular hardware.

The severity of the virtualization impact on the VM performance is influenced by a variety of factors, which include:

  • The number of concurrently running VMs.
  • The amount of virtual devices used by each VM.
  • The device types used by the VMs.

Reducing VM performance loss
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RHEL 10 provides a number of features you can use to reduce the negative performance effects of virtualization. Notably:

  • The TuneD service can automatically optimize the resource distribution and performance of your VMs.
  • Block I/O tuning can improve the performances of the VM’s block devices, such as disks.
  • NUMA tuning can increase vCPU performance.
  • Virtual networking can be optimized in various ways.
Important

Tuning VM performance can have negative effects on other virtualization functions. For example, it can make migrating the modified VM more difficult.