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Understanding CPU Pinning in KVM

In the world of virtualization, it is important to ensure optimal performance and resource allocation.

KVM, short for Kernel-based Virtual Machine offers a powerful solution for virtualization on Linux systems. Additionally, it offers CPU pinning as a feature.

CPU pinning involves binding a VM’s virtual CPUs to specific physical CPU cores or threads. This offers greater control over resource allocation and performance. Today, we are going to explore CPU pinning in KVM and how to set it up.

Why Use CPU Pinning in KVM?

CPU pinning offers several benefits for our virtualized environment.

For instance, CPU pinning ensures that a VM’s execution remains confined to the assigned CPU cores or threads. In other words, it prevents dynamic scheduling across different physical cores. This predictability is important for latency-sensitive workloads or applications that need dedicated CPU resources.

Furthermore, CPU pinning allows us to allocate specific CPU cores exclusively to a VM. Thereby, providing isolation from other VMs or processes running on the host machine.

This isolation prevents resource contention and boosts overall performance.

How to Perform CPU Pinning in KVM

1. First, we have to determine the exact core IDs we want to assign to the VM. We can use tools like numactl to identify the NUMA nodes and the core IDs associated with each one.
2. Then, we must locate the XML configuration file of the VM and open it. It is usually found in the /etc/libvirt/qemu/ directory.
3. Next, find the line following line in the file:

   <vcpu placement='static'>

   This line mentions the number of vCPUs assigned to the VM.

   We have to edit this line as seen below:

   <vcpu placement='static' cpuset='0,4,8,12,16,20,24,28'>8</vcpu>

   Here, we have included the cpuset attribute to indicate the desired core IDs to be allocated.
4. Now, save the changes and restart libvirtd.
5. Finally, it is time time to start the VM. We will be able to monitor the CPU cores utilized by the VM using tools like htop or other system monitoring commands.

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Conclusion

In summary, our Support Techs introduced us to CPU pinning in KVM and demonstrated how to set it up.