KVM Performance Tuning

The following sections contain VM performance tuning recommendations to improve system performance. These performance recommendations are general guidelines and are not exhaustive. Refer to the documentation provided by your Linux OS and KVM host vendors. For example, Redhat provides extensive documentation on using virt-manager and optimizing VM performance. Refer to the Redhat Virtualization Tuning and Optimization Guide for details.

General Recommendations

• Ensure the number of vCPUs in an instance is always an even number (4, 6, 8, and so on) as hyper threaded vcpus are used.
• For best performance, make sure a single instance is confined to a single NUMA. Performance degradation occurs if an instance spans across multiple NUMAs.
• Ensure the physical NICs associated with an instance are connected to the same NUMA/socket where the instance is hosted. Doing so reduces the remote node memory access, which in turn helps improve the performance.

Recommended BIOS Settings

Processor and CPU Details

To determine the host system's processor and CPU details, perform the following steps:

1. Execute the following command to determine how many vCPUs are assigned to host CPUs:

   lscpu -p

   The command provides the following output:

   CPU Architecture

   

   The first column lists the logical CPU number of a CPU as used by the Linux kernel. The second column lists the logical core number - use this information for vCPU pinning.

Persistent CPU Pinning

CPU pinning ensures that a VM only gets CPU time from a specific CPU or set of CPUs. Pinning is performed on each logical CPU of the guest VM against each core ID in the host system. The CPU pinning information is lost every time the VM instance is shutdown or restarted. To avoid entering the pinning information again, update the KVM configuration XML file on the host system.

 Use the following steps to update the pinning information in the KVM configuration XML file:

1. Shutdown the VM instance.

2. Enter the following command.

   virsh

   The command provides the following output:

   virsh Prompt

   

3. Enter the following command to edit the VM instance:

   virsh # edit <KVM_instance_name>

4. Search for the vcpu placement attribute.

   vCPU Placement Attribute

   

5. Enter CPU pinning information as shown below:

   CPU Pinning Information

   
   Tip

   Ensure that no two VM instances have the same physical core affinity. For example, if VM1 has affinity of 0,1,2,3 assigned, then ensure no VM is pinned to 0,1,2,3,8,9,10 or 11 as these CPUs belong to the physical core assigned to VM1. Also, assign all other VM instances running on the same host with affinity; otherwise the VMs without affinity may impact the performance of VMs that have affinity.

6. Enter the following command to save and exit the XML file.

   :wq

CPU Mode Configuration

Use the following steps to edit the VM CPU mode:

1. Shutdown the VM instance.

2. Enter the following command.

   virsh

   The following output displays:

   virsh Prompt

   

3. Enter the following command to edit the VM instance:

   edit <KVM_instance_name>

4. Search for the cpu mode attribute.

   cpu mode

   

5. Edit the cpu mode attribute with the following:

   Editing CPU Mode

   
   Tip

   Ensure the topology details entered are identical to the topology details set while creating the VM instance. For example, if the topology was set to 1 socket, 2 cores and 2 threads, enter the same details in this XML file.

6. Enter the following command to save and exit the XML file.

   :wq

7. Enter the following command to start the VM instance.

   start <KVM_instance_name>

Increasing the Transmit Queue Length for virt-io Interfaces

This section is applicable only for virt-io based interfaces. 

By default, the transmit queue length is set to 500. To increase the transmit queue length to 4096, use the following procedure:

1. Execute the following command to identify the available interfaces:

   virsh

   The virsh prompt displays.

2. Execute the following command.

   domiflist <VM_instance_name>

   The list of active interfaces displays.

   Active Interfaces List

   

3. Execute the following command to increase the transmit queue lengths for the tap interfaces.

   ifconfig <interface_name> txqueuelen <length>

   The interface_name is the name of the interface you want to change, and length is the new queue length. For example, ifconfig macvtap4 txqueuelen 4096.

4. Execute the following command to verify the value of the interface length.

   ifconfig <interface_name>

   The command provides the following output.

   Interface Information

   

Kernel Same-page Metering (KSM) Settings

Kernel same-page metering (KSM) is a technology which finds common memory pages inside a Linux system and merges the pages to save memory resources. In the event of one of the copies being updated, a new copy is created so the function is transparent to the processes on the system. For hypervisors, KSM is highly beneficial when multiple guests are running with the same level of the operating system. However, there is overhead due to the scanning process which may cause the applications to run slower, which is not desirable.

Turn off KSM in the host.

Deactivate KSM by stopping the ksmtuned and the ksm services as shown below. This does not persist across reboots.

# systemctl stop ksm
# systemctl stopksmtuned

Disable KSM persistently as shown below:

# systemctl disable ksm
# systemctl disable ksmtuned

Host Pinning

To avoid performance impact on VMs due to host-level Linux services, host pinning isolates physical cores where a guest VM is hosted from physical cores where the Linux host processes/services run. 

In this example, the core 0 (Core 0 and core 36 are logical cores) and core 1 (Core 1 and core 37 are logical cores) are reserved for Linux host processes.

The CPUAffinity option in /etc/systemd/system.conf sets affinity to systemd by default, as well as for everything it launches, unless their .service file overrides the CPUAffinity setting with its own value. Configure the CPUAffinity option in /etc/systemd/system.conf.

Execute the following command:

lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                72
On-line CPU(s) list:   0-71
Thread(s) per core:    2
Core(s) per socket:    18
Socket(s):             2
NUMA node(s):          2
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 79
Model name:            Intel(R) Xeon(R) CPU E5-2697 v4 @ 2.30GHz
Stepping:              1
CPU MHz:               2699.984
BogoMIPS:              4604.99
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              46080K
NUMA node0 CPU(s):     0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58,60,62,64,66,68,70
NUMA node1 CPU(s):     1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59,61,63,65,67,69,71

To dedicate the physical CPUs 0 and 1 for host processing, in the file /etc/systemd/system.conf, specify CPUAffinity as 0 1 36 37, as shown below. Restart the system.

CPUAffinity=0 1 36 37

Back Up VMs with 1G hugepages

The number of hugepages is decided based on the total memory available on the host. 

1. Configure the huge page size as 1G and number of huge pages by appending the following line to the kernel command line options in /etc/default/grub. In the example below, the host  has a total of 256G memory, out of which 200G is configured as hugepages. 

   GRUB_TIMEOUT=5
   
   GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
   
   GRUB_DEFAULT=saved
   
   GRUB_DISABLE_SUBMENU=true
   
   GRUB_TERMINAL_OUTPUT="console"
   
   GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8 crashkernel=auto intel_iommu=on iommu=pt default_hugepagesz=1GB hugepagesz=1G hugepages=200 rhgb quiet"
   
   GRUB_DISABLE_RECOVERY="true"

2. Regenerate the GRUB2 configuration as shown below: 

   1. If your system uses BIOS firmware, execute the following command:

      # grub2-mkconfig -o /boot/grub2/grub.cfg

   2. On a system with UEFI firmware, execute the following command: 

      # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

3. Add lines in your instance XML file using virsh edit <instanceName>. The example below is for a 32G instance: 

   <memory unit='KiB'>33554432</memory>
   
   <currentMemory unit='KiB'>33554432</currentMemory>
   
   <memoryBacking>
   
   <hugepages>
   
   <page size='1048576' unit='KiB' nodeset='0'/>
   
   </hugepages>
   
   </memoryBacking>

   The previous example pins the VM on NUMA node 0. For hosting a second VM on NUMA node 1, use nodeset = ‘1’.

4. Restart the host.

5. Obtain the PID of the VM from the following command:

   ps -eaf | grep qemu | grep -i <vm_name>

6. Execute the following command to verify VM memory is received from a single NUMA node:

   numastat -p  <vmpid>

Disable Flow Control

Use the following steps to disable flow control:

1. Log in to the system as the root user.

2. Execute the following command to disable flow control for interfaces attached to the SWe VM.

   ethtool -A <interface name> rx off tx off autoneg off  

   Tip

   Use the <interface name> from the actual configuration.

   Example:

   ethtool -A p4p3 rx off tx off autoneg off
ethtool -A p4p4 rx off tx off autoneg off
ethtool -A em3 rx off tx off autoneg off
ethtool -A em4 rx off tx off autoneg off