18.6. Optimizing virtual machine CPU performance

Procedure

1. Optional: View the current state of the vCPUs in the selected VM. For example, to display the number of vCPUs on the testguest VM:

   # virsh vcpucount testguest
   maximum      config         4
   maximum      live           2
   current      config         2
   current      live           1

   This output indicates that testguest is currently using 1 vCPU, and 1 more vCPu can be hot plugged to it to increase the VM’s performance. However, after reboot, the number of vCPUs testguest uses will change to 2, and it will be possible to hot plug 2 more vCPUs.

2. Adjust the maximum number of vCPUs that can be attached to the VM, which takes effect on the VM’s next boot.

   For example, to increase the maximum vCPU count for the testguest VM to 8:

   # virsh setvcpus testguest 8 --maximum --config

   Note that the maximum may be limited by the CPU topology, host hardware, the hypervisor, and other factors.

3. Adjust the current number of vCPUs attached to the VM, up to the maximum configured in the previous step. For example:

   • To increase the number of vCPUs attached to the running testguest VM to 4:

     # virsh setvcpus testguest 4 --live

     This increases the VM’s performance and host load footprint of testguest until the VM’s next boot.

   • To permanently decrease the number of vCPUs attached to the testguest VM to 1:

     # virsh setvcpus testguest 1 --config

     This decreases the VM’s performance and host load footprint of testguest after the VM’s next boot. However, if needed, additional vCPUs can be hot plugged to the VM to temporarily increase its performance.

Procedure

1. Log in to the RHEL 10 web console.

2. In the Virtual Machines interface, click the VM whose information you want to see.

   A new page opens with an Overview section with basic information about the selected VM and a Console section to access the VM’s graphical interface.

3. Click edit next to the number of vCPUs in the Overview pane.

   The vCPU details dialog appears.

4. Configure the virtual CPUs for the selected VM.

   • vCPU Count - The number of vCPUs currently in use.

     注意

     The vCPU count cannot be greater than the vCPU Maximum.

   • vCPU Maximum - The maximum number of virtual CPUs that can be configured for the VM. If this value is higher than the vCPU Count, additional vCPUs can be attached to the VM.
   • Sockets - The number of sockets to expose to the VM.
   • Cores per socket - The number of cores for each socket to expose to the VM.

   • Threads per core - The number of threads for each core to expose to the VM.

     重要

     Note that the Sockets, Cores per socket, and Threads per core options adjust the CPU topology of the VM. This may be beneficial for vCPU performance and may impact the functionality of certain software in the guest OS. If a different setting is not required by your deployment, keep the default values.

5. Click Apply.

   The virtual CPUs for the VM are configured.

6. If the VM is running, restart it for the changes to virtual CPU settings to take effect.

Procedure

1. Set the VM’s NUMA policy to Preferred. For example, to configure the testguest5 VM:

   # virt-xml testguest5 --edit --vcpus placement=auto
   # virt-xml testguest5 --edit --numatune mode=preferred

2. Set up the numad service to automatically align the VM CPU with memory resources.

   # echo 1 > /proc/sys/kernel/numa_balancing

3. Start the numad service.

   # systemctl start numad

4. Optional: Tune NUMA settings manually. Specify which host NUMA nodes will be assigned specifically to a certain VM. This can improve the host memory usage by the VM’s vCPU.

   1. Use the numactl command to view the NUMA topology on the host:

      # numactl --hardware
      
      available: 2 nodes (0-1)
      node 0 size: 18156 MB
      node 0 free: 9053 MB
      node 1 size: 18180 MB
      node 1 free: 6853 MB
      node distances:
      node   0   1
        0:  10  20
        1:  20  10

   2. Edit the XML configuration of a VM to assign CPU and memory resources to specific NUMA nodes. For example, the following configuration sets testguest6 to use vCPUs 0-7 on NUMA node 0 and vCPUS 8-15 on NUMA node 1. Both nodes are also assigned 16 GiB of VM’s memory.

      # virsh edit <testguest6>
      
      <domain type='kvm'>
        <name>testguest6</name>
        ...
        <vcpu placement='static'>16</vcpu>
        ...
        <cpu ...>
          <numa>
            <cell id='0' cpus='0-7' memory='16' unit='GiB'/>
            <cell id='1' cpus='8-15' memory='16' unit='GiB'/>
          </numa>
        ...
      </domain>

      注意

      For best performance results, it is a good practice to respect the maximum memory size for each NUMA node on the host.

   3. If the VM is running, restart it to apply the configuration.

Procedure

1. Check the CPU topology on the host:

   # lscpu -p=node,cpu
   
   Node,CPU
   0,0
   0,1
   0,2
   0,3
   0,4
   0,5
   0,6
   0,7
   1,0
   1,1
   1,2
   1,3
   1,4
   1,5
   1,6
   1,7

   In this example, the output contains NUMA nodes and the available physical CPU threads on the host.

2. Check the number of vCPU threads inside the VM:

   # lscpu -p=node,cpu
   
   Node,CPU
   0,0
   0,1
   0,2
   0,3

   In this example, the output contains NUMA nodes and the available vCPU threads inside the VM.

3. Pin specific vCPU threads from a VM to a specific host CPU or range of CPUs. This is suggested as a safe method of vCPU performance improvement.

   For example, the following commands pin vCPU threads 0 to 3 of the testguest6 VM to host CPUs 1, 3, 5, 7, respectively:

   # virsh vcpupin testguest6 0 1
   # virsh vcpupin testguest6 1 3
   # virsh vcpupin testguest6 2 5
   # virsh vcpupin testguest6 3 7

4. Optional: Verify whether the vCPU threads are successfully pinned to CPUs.

   # virsh vcpupin testguest6
   VCPU   CPU Affinity
   ----------------------
   0      1
   1      3
   2      5
   3      7

5. Optional: After pinning vCPU threads, you can also pin QEMU process threads associated with a specified VM to a specific host CPU or range of CPUs. This can further help the QEMU process to run more efficiently on the physical CPU.

   For example, the following commands pin the QEMU process thread of testguest6 to CPUs 2 and 4, and verify this was successful:

   # virsh emulatorpin testguest6 2,4
   # virsh emulatorpin testguest6
   emulator: CPU Affinity
   ----------------------------------
          *: 2,4

Procedure

1. View the current vCPU scheduling configuration on the host.

   # virsh schedinfo <vm_name>
   
   Scheduler      : posix
   cpu_shares     : 0
   vcpu_period : 0
   vcpu_quota : 0
   emulator_period: 0
   emulator_quota : 0
   global_period  : 0
   global_quota   : 0
   iothread_period: 0
   iothread_quota : 0

2. To configure an absolute vCPU cap for a VM, set the vcpu_period and vcpu_quota parameters. Both parameters use a numerical value that represents a time duration in microseconds.

   1. Set the vcpu_period parameter by using the virsh schedinfo command. For example:

      # virsh schedinfo <vm_name> --set vcpu_period=100000

      In this example, the vcpu_period is set to 100,000 microseconds, which means the scheduler enforces vCPU capping during this time interval.

      You can also use the --live --config options to configure a running VM without restarting it.

   2. Set the vcpu_quota parameter by using the virsh schedinfo command. For example:

      # virsh schedinfo <vm_name> --set vcpu_quota=50000

      In this example, the vcpu_quota is set to 50,000 microseconds, which specifies the maximum amount of CPU time that the VM can use during the vcpu_period time interval. In this case, vcpu_quota is set as the half of vcpu_period, so the VM can use up to 50% of the CPU time during that interval.

      You can also use the --live --config options to configure a running VM without restarting it.

Procedure

1. Check the current CPU weight of a VM.

   # virsh schedinfo <vm_name>
   
   Scheduler      : posix
   cpu_shares : 1024
   vcpu_period    : 0
   vcpu_quota     : 0
   emulator_period: 0
   emulator_quota : 0
   global_period  : 0
   global_quota   : 0
   iothread_period: 0
   iothread_quota : 0

2. Adjust the CPU weight to a preferred value.

   # virsh schedinfo <vm_name> --set cpu_shares=2048
   
   Scheduler      : posix
   cpu_shares : 2048
   vcpu_period    : 0
   vcpu_quota     : 0
   emulator_period: 0
   emulator_quota : 0
   global_period  : 0
   global_quota   : 0
   iothread_period: 0
   iothread_quota : 0

   In this example, cpu_shares is set to 2048. This means that if all other VMs have the value set to 1024, this VM gets approximately twice the amount of CPU time.

   You can also use the --live --config options to configure a running VM without restarting it.

Procedure

1. Enable KSM:

   警告

   Enabling KSM increases CPU utilization and affects overall CPU performance.

   1. Install the ksmtuned service:

      # dnf install ksmtuned

   2. Start the service:

      • To enable KSM for a single session, use the systemctl utility to start the ksm and ksmtuned services.

        # systemctl start ksm
        # systemctl start ksmtuned

      • To enable KSM persistently, use the systemctl utility to enable the ksm and ksmtuned services.

        # systemctl enable ksm
        Created symlink /etc/systemd/system/multi-user.target.wants/ksm.service  /usr/lib/systemd/system/ksm.service
        
        # systemctl enable ksmtuned
        Created symlink /etc/systemd/system/multi-user.target.wants/ksmtuned.service  /usr/lib/systemd/system/ksmtuned.service

2. Monitor the performance and resource consumption of VMs on your host to evaluate the benefits of activating KSM. Specifically, ensure that the additional CPU usage by KSM does not offset the memory improvements and does not cause additional performance issues. In latency-sensitive workloads, also pay attention to cross-NUMA page merges.

3. Optional: If KSM has not improved your VM performance, disable it:

   • To disable KSM for a single session, use the systemctl utility to stop ksm and ksmtuned services.

     # systemctl stop ksm
     # systemctl stop ksmtuned

   • To disable KSM persistently, use the systemctl utility to disable ksm and ksmtuned services.

     # systemctl disable ksm
     Removed /etc/systemd/system/multi-user.target.wants/ksm.service.
     # systemctl disable ksmtuned
     Removed /etc/systemd/system/multi-user.target.wants/ksmtuned.service.

     注意

     Memory pages shared between VMs before deactivating KSM will remain shared. To stop sharing, delete all the PageKSM pages in the system by using the following command:

     # echo 2 > /sys/kernel/mm/ksm/run

     However, this command increases memory usage, and might cause performance problems on your host or your VMs.