8.9. Improving Guest Virtual Machine Response Time

Guest virtual machines can sometimes be slow to respond with certain workloads and usage patterns. Examples of situations which may cause slow or unresponsive guest virtual machines:

KVM guest virtual machines function as Linux processes. Linux processes are not permanently kept in main memory (physical RAM) and will be placed into swap space (virtual memory) especially if they are not being used. If a guest virtual machine is inactive for long periods of time, the host physical machine kernel may move the guest virtual machine into swap. As swap is slower than physical memory it may appear that the guest is not responding. This changes once the guest is loaded into the main memory. Note that the process of loading a guest virtual machine from swap to main memory may take several seconds per gigabyte of RAM assigned to the guest virtual machine, depending on the type of storage used for swap and the performance of the components.

KVM guest virtual machines processes may be moved to swap regardless of whether memory is overcommitted or overall memory usage.

Using unsafe overcommit levels or overcommitting with swap turned off guest virtual machine processes or other critical processes is not recommended. Always ensure the host physical machine has sufficient swap space when overcommitting memory.

For more information on overcommitting with KVM, refer to Chapter 6, Overcommitting with KVM.