32. VM Power Management Application

32.1. Introduction

Applications running in Virtual Environments have an abstract view of the underlying hardware on the Host, in particular applications cannot see the binding of virtual to physical hardware. When looking at CPU resourcing, the pinning of Virtual CPUs(vCPUs) to Host Physical CPUs(pCPUS) is not apparent to an application and this pinning may change over time. Furthermore, Operating Systems on virtual machines do not have the ability to govern their own power policy; the Machine Specific Registers (MSRs) for enabling P-State transitions are not exposed to Operating Systems running on Virtual Machines(VMs).

The Virtual Machine Power Management solution shows an example of how a DPDK application can indicate its processing requirements using VM local only information(vCPU/lcore) to a Host based Monitor which is responsible for accepting requests for frequency changes for a vCPU, translating the vCPU to a pCPU via libvirt and affecting the change in frequency.

The solution is comprised of two high-level components:

  1. Example Host Application

    Using a Command Line Interface(CLI) for VM->Host communication channel management allows adding channels to the Monitor, setting and querying the vCPU to pCPU pinning, inspecting and manually changing the frequency for each CPU. The CLI runs on a single lcore while the thread responsible for managing VM requests runs on a second lcore.

    VM requests arriving on a channel for frequency changes are passed to the librte_power ACPI cpufreq sysfs based library. The Host Application relies on both qemu-kvm and libvirt to function.

  2. librte_power for Virtual Machines

    Using an alternate implementation for the librte_power API, requests for frequency changes are forwarded to the host monitor rather than the APCI cpufreq sysfs interface used on the host.

    The l3fwd-power application will use this implementation when deployed on a VM (see Chapter 11 “L3 Forwarding with Power Management Application”).

Figure 24. Highlevel Solution


32.2. Overview

VM Power Management employs qemu-kvm to provide communications channels between the host and VMs in the form of Virtio-Serial which appears as a paravirtualized serial device on a VM and can be configured to use various backends on the host. For this example each Virtio-Serial endpoint on the host is configured as AF_UNIX file socket, supporting poll/select and epoll for event notification. In this example each channel endpoint on the host is monitored via epoll for EPOLLIN events. Each channel is specified as qemu-kvm arguments or as libvirt XML for each VM, where each VM can have a number of channels up to a maximum of 64 per VM, in this example each DPDK lcore on a VM has exclusive access to a channel.

To enable frequency changes from within a VM, a request via the librte_power interface is forwarded via Virtio-Serial to the host, each request contains the vCPU and power command(scale up/down/min/max). The API for host and guest librte_power is consistent across environments, with the selection of VM or Host Implementation determined at automatically at runtime based on the environment.

Upon receiving a request, the host translates the vCPU to a pCPU via the libvirt API before forwarding to the host librte_power.

Figure 25. VM request to scale frequency


32.2.1. Performance Considerations

While Haswell Microarchitecture allows for independent power control for each core, earlier Microarchtectures do not offer such fine grained control. When deployed on pre-Haswell platforms greater care must be taken in selecting which cores are assigned to a VM, for instance a core will not scale down until its sibling is similarly scaled.

32.3. Configuration

32.3.1. BIOS

Enhanced Intel SpeedStep® Technology must be enabled in the platform BIOS if the power management feature of DPDK is to be used. Otherwise, the sys file folder /sys/devices/system/cpu/cpu0/cpufreq will not exist, and the CPU frequency-based power management cannot be used. Consult the relevant BIOS documentation to determine how these settings can be accessed.

32.3.2. Host Operating System

The Host OS must also have the apci_cpufreq module installed, in some cases the intel_pstate driver may be the default Power Management environment. To enable acpi_cpufreq and disable intel_pstate, add the following to the grub linux command line:


Upon rebooting, load the acpi_cpufreq module:

modprobe acpi_cpufreq

32.3.3. Hypervisor Channel Configuration

Virtio-Serial channels are configured via libvirt XML:

<controller type='virtio-serial' index='0'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
<channel type='unix'>
  <source mode='bind' path='/tmp/powermonitor/{vm_name}.{channel_num}'/>
  <target type='virtio' name='virtio.serial.port.poweragent.{vm_channel_num}/>
  <address type='virtio-serial' controller='0' bus='0' port='{N}'/>

Where a single controller of type virtio-serial is created and up to 32 channels can be associated with a single controller and multiple controllers can be specified. The convention is to use the name of the VM in the host path {vm_name} and to increment {channel_num} for each channel, likewise the port value {N} must be incremented for each channel.

Each channel on the host will appear in path, the directory /tmp/powermonitor/ must first be created and given qemu permissions

mkdir /tmp/powermonitor/
chown qemu:qemu /tmp/powermonitor

Note that files and directories within /tmp are generally removed upon rebooting the host and the above steps may need to be carried out after each reboot.

The serial device as it appears on a VM is configured with the target element attribute name and must be in the form of virtio.serial.port.poweragent.{vm_channel_num}, where vm_channel_num is typically the lcore channel to be used in DPDK VM applications.

Each channel on a VM will be present at /dev/virtio-ports/virtio.serial.port.poweragent.{vm_channel_num}

32.4. Compiling and Running the Host Application

32.4.1. Compiling

  1. export RTE_SDK=/path/to/rte_sdk
  2. cd ${RTE_SDK}/examples/vm_power_manager
  3. make

32.4.2. Running

The application does not have any specific command line options other than EAL:

./build/vm_power_mgr [EAL options]

The application requires exactly two cores to run, one core is dedicated to the CLI, while the other is dedicated to the channel endpoint monitor, for example to run on cores 0 & 1 on a system with 4 memory channels:

./build/vm_power_mgr -c 0x3 -n 4

After successful initialisation the user is presented with VM Power Manager CLI:


Virtual Machines can now be added to the VM Power Manager:

vm_power> add_vm {vm_name}

When a {vm_name} is specified with the add_vm command a lookup is performed with libvirt to ensure that the VM exists, {vm_name} is used as an unique identifier to associate channels with a particular VM and for executing operations on a VM within the CLI. VMs do not have to be running in order to add them.

A number of commands can be issued via the CLI in relation to VMs:

Remove a Virtual Machine identified by {vm_name} from the VM Power Manager.

rm_vm {vm_name}

Add communication channels for the specified VM, the virtio channels must be enabled in the VM configuration(qemu/libvirt) and the associated VM must be active. {list} is a comma-separated list of channel numbers to add, using the keyword ‘all’ will attempt to add all channels for the VM:

add_channels {vm_name} {list}|all

Enable or disable the communication channels in {list}(comma-separated) for the specified VM, alternatively list can be replaced with keyword ‘all’. Disabled channels will still receive packets on the host, however the commands they specify will be ignored. Set status to ‘enabled’ to begin processing requests again:

set_channel_status {vm_name} {list}|all enabled|disabled

Print to the CLI the information on the specified VM, the information lists the number of vCPUS, the pinning to pCPU(s) as a bit mask, along with any communication channels associated with each VM, along with the status of each channel:

show_vm {vm_name}

Set the binding of Virtual CPU on VM with name {vm_name} to the Physical CPU mask:

set_pcpu_mask {vm_name} {vcpu} {pcpu}

Set the binding of Virtual CPU on VM to the Physical CPU:

set_pcpu {vm_name} {vcpu} {pcpu}

Manual control and inspection can also be carried in relation CPU frequency scaling:

Get the current frequency for each core specified in the mask:

show_cpu_freq_mask {mask}

Set the current frequency for the cores specified in {core_mask} by scaling each up/down/min/max:

set_cpu_freq {core_mask} up|down|min|max

Get the current frequency for the specified core:

show_cpu_freq {core_num}

Set the current frequency for the specified core by scaling up/down/min/max:

set_cpu_freq {core_num} up|down|min|max

32.5. Compiling and Running the Guest Applications

For compiling and running l3fwd-power, see Chapter 11 “L3 Forwarding with Power Management Application”.

A guest CLI is also provided for validating the setup.

For both l3fwd-power and guest CLI, the channels for the VM must be monitored by the host application using the add_channels command on the host.

32.5.1. Compiling

  1. export RTE_SDK=/path/to/rte_sdk
  2. cd ${RTE_SDK}/examples/vm_power_manager/guest_cli
  3. make

32.5.2. Running

The application does not have any specific command line options other than EAL:

./build/vm_power_mgr [EAL options]

The application for example purposes uses a channel for each lcore enabled, for example to run on cores 0,1,2,3 on a system with 4 memory channels:

./build/guest_vm_power_mgr -c 0xf -n 4

After successful initialisation the user is presented with VM Power Manager Guest CLI:


To change the frequency of a lcore, use the set_cpu_freq command. Where {core_num} is the lcore and channel to change frequency by scaling up/down/min/max.

set_cpu_freq {core_num} up|down|min|max