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If available for operating systems where the utility tuned-admin exists (most recent releases of Red Hat derived O/Ses), the virtual-host setting is very beneficial for guest VM responsivity.

• To verify current settings:

tuned-adm active (output should be virtual-host)

• To set this in the running kernel:

tuned-adm profile virtual-host

• To set this permanently on Red Hat systems:

echo virtual-host > /etc/tuned/active_profile

Proper performance of the DSC requires full cores and not hyperthreaded cores.  When pinning, use only Processor IDs that represent actual cores, not hyperthreaded cores.

If you do not statically pin vCPUs to physical CPUs, your system may incur a significant degradation in performance.

Using the KVM GUI software to set the CPU pinning is prone to errors. Certain versions of the virt-manager software do not provide full support for the CPU pinning function. It is recommended that you verify CPU pinning on your system using the Linux system command (virsh) in a console window, for example:

  # virsh vcpupin <VM_name>

  VCPU: CPU Affinity
  ----------------------------------
  0: 12
  1: 13
  2: 14
  3: 15
  4: 16
  5: 17

In the above example, the output indicates six vCPUs of <VM_name> are each assigned to one physical CPU device. The CPU mapping begins with vCPU_0 pinned to physical CPU_12; and continues to vCPU_5 pinned to physical CPU_17.

# virsh emulatorpin <VM_name> --cpulist 0-3 --config --current

In a multi-processor system, NUMA allows some CPUs to access system resources faster than other CPUs. In a Linux OS, system resources are split up into NUMA domains.

NUMA domains are used in virt-manager to set the CPU Pinning option of the VM.

To identify NUMA domains on the KVM Host, run the command, lscpu (Linux RedHat OS), at the system prompt. For example, on a KVM Host running a Linux RedHat OS, the command returns the following:

Example:
lscpu NUMA node0 CPU(s):     0-5,12-17 NUMA node1 CPU(s):     6-11,18-23

A VM uses special features available on a physical CPU device if it is allowed to identify all CPU resources (and their features) on the KVM Host. Host CPU visibility can be enabled in virt-manager using the 'Copy Host CPU Configuration' button available in the Processor Configuration screen. The following figure shows virt-manager's Processor Configuration screen and the location of the 'Copy host CPU configuration' button.

The VM CPU and emulator pinning can be configured at install time using the installation file (refer to The VM Configuration File).

Hyperthreading adds 15-25% to the performance of a CPU core. Processor IDs that represent Hyperthreading are not considered a full CPU core.

To identify CPUs with Hyperthreading, login to the KVM Host and execute the Bash command 'cat /proc/cpuinfo'. The command returns information about physical CPUs and CPU cores available on the KVM Host. The command list shows each CPU and CPU core on the Host which are identified by a Processor ID (command output format: processor    : n). CPU cores that belong to the same physical CPU appear in the listing with the same Processor ID.

Two CPU cores, each with different Processor ID's, but with the same Physical ID and Core ID, is the Linux representation of a CPU core with Hyperthreading capability.

For example, the following 'cat /proc/cpuinfo' command listing shows there are two CPUs with six cores each. All CPUs support hyperthreading. Processor 0 and 12 represent a CPU core and its hyperthreading capability.

Example:
bash# cat /proc/cpuinfo
# cat /proc/cpuinfo | grep -e "^processor" -e "^physical" -e "^core id"
processor       : 0
physical id     : 0
core id         : 0
processor       : 1
physical id     : 0
core id         : 1
[snip]
processor       : 12
physical id     : 0
core id         : 0

Recent CPU devices allow CPU Frequency Scaling to reduce power consumption when only partial CPU resources are required. Under heavy traffic (or traffic bursts) conditions, the DSC SWe VM performs better (that is, maintains a higher throughput with less latency) if CPU Frequency Scaling is disabled and the CPU cores are permitted to run at full speed all the time.

To disable CPU Frequency Scaling, run the following Bash command on the KVM Host for each CPU running a VM :

Example for cpu0 and cpu1:
bash# echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
bash# echo performance > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor

On systems running a RedHat LinuxOS, the above command is used to set the CPU Frequency Scaling option, which can be enabled at runtime for specific Processor IDs.

The Frequency Scaling Governor setting does not persist across KVM Host reboots. If the KVM Host is running a RedHat OS variant, this setting can be made persistent by adding the line(s) to /etc/rc.d/rc.local. For further details, please consult the support documentation for your Linux OS on the KVM Host.

Some general recommendations are presented in this section, however, the list is not exhaustive. For more information on VM performance tuning, please consult your Linux OS and KVM Host vendor documentation.