2. Power Management

The DPDK Power Management feature allows users space applications to save power by dynamically adjusting CPU frequency or entering into different C-States.

  • Adjusting the CPU frequency dynamically according to the utilization of RX queue.

  • Entering into different deeper C-States according to the adaptive algorithms to speculate brief periods of time suspending the application if no packets are received.

The interfaces for adjusting the operating CPU frequency are in the power management library. C-State control is implemented in applications according to the different use cases.

2.1. CPU Frequency Scaling

The Linux kernel provides a cpufreq module for CPU frequency scaling for each lcore. For example, for cpuX, /sys/devices/system/cpu/cpuX/cpufreq/ has the following sys files for frequency scaling:

  • affected_cpus

  • bios_limit

  • cpuinfo_cur_freq

  • cpuinfo_max_freq

  • cpuinfo_min_freq

  • cpuinfo_transition_latency

  • related_cpus

  • scaling_available_frequencies

  • scaling_available_governors

  • scaling_cur_freq

  • scaling_driver

  • scaling_governor

  • scaling_max_freq

  • scaling_min_freq

  • scaling_setspeed

In the DPDK, scaling_governor is configured in user space. Then, a user space application can prompt the kernel by writing scaling_setspeed to adjust the CPU frequency according to the strategies defined by the user space application.

2.2. Core-load Throttling through C-States

Core state can be altered by speculative sleeps whenever the specified lcore has nothing to do. In the DPDK, if no packet is received after polling, speculative sleeps can be triggered according the strategies defined by the user space application.

2.3. Per-core Turbo Boost

Individual cores can be allowed to enter a Turbo Boost state on a per-core basis. This is achieved by enabling Turbo Boost Technology in the BIOS, then looping through the relevant cores and enabling/disabling Turbo Boost on each core.

2.4. Use of Power Library in a Hyper-Threaded Environment

In the case where the power library is in use on a system with Hyper-Threading enabled, the frequency on the physical core is set to the highest frequency of the Hyper-Thread siblings. So even though an application may request a scale down, the core frequency will remain at the highest frequency until all Hyper-Threads on that core request a scale down.

2.5. API Overview of the Power Library

The main methods exported by power library are for CPU frequency scaling and include the following:

  • Freq up: Prompt the kernel to scale up the frequency of the specific lcore.

  • Freq down: Prompt the kernel to scale down the frequency of the specific lcore.

  • Freq max: Prompt the kernel to scale up the frequency of the specific lcore to the maximum.

  • Freq min: Prompt the kernel to scale down the frequency of the specific lcore to the minimum.

  • Get available freqs: Read the available frequencies of the specific lcore from the sys file.

  • Freq get: Get the current frequency of the specific lcore.

  • Freq set: Prompt the kernel to set the frequency for the specific lcore.

  • Enable turbo: Prompt the kernel to enable Turbo Boost for the specific lcore.

  • Disable turbo: Prompt the kernel to disable Turbo Boost for the specific lcore.

2.6. User Cases

The power management mechanism is used to save power when performing L3 forwarding.

2.7. PM QoS

The /sys/devices/system/cpu/cpuX/power/pm_qos_resume_latency_us sysfs interface is used to set and get the resume latency limit on the cpuX for userspace. Each cpuidle governor in Linux selects which idle state to enter based on this CPU resume latency in their idle task.

The deeper the idle state, the lower the power consumption, but the longer the resume time. Some services are latency sensitive and request a low resume time, like interrupt packet receiving mode.

Applications can set and get the CPU resume latency with rte_power_qos_set_cpu_resume_latency() and rte_power_qos_get_cpu_resume_latency() respectively. Applications can set a strict resume latency (zero value) to lower the resume latency and get better performance (instead, the power consumption of platform may increase).

2.8. Ethernet PMD Power Management API

2.8.1. Abstract

Existing power management mechanisms require developers to change application design or change code to make use of it. The PMD power management API provides a convenient alternative by utilizing Ethernet PMD RX callbacks, and triggering power saving whenever empty poll count reaches a certain number.

  • Monitor

    This power saving scheme will put the CPU into optimized power state and monitor the Ethernet PMD RX descriptor address, waking the CPU up whenever there’s new traffic. Support for this scheme may not be available on all platforms, and further limitations may apply (see below).

  • Pause

    This power saving scheme will avoid busy polling by either entering power-optimized sleep state with rte_power_pause() function, or, if it’s not supported by the underlying platform, use rte_pause().

  • Frequency scaling

    This power saving scheme will use librte_power library functionality to scale the core frequency up/down depending on traffic volume. The reaction time of the frequency scaling mode is longer than the pause and monitor mode.

The “monitor” mode is only supported in the following configurations and scenarios:

  • On Linux* x86_64, rte_power_monitor() requires WAITPKG instruction set being supported by the CPU, while rte_power_monitor_multi() requires WAITPKG and RTM instruction sets being supported by the CPU. RTM instruction set may also require booting the Linux with tsx=on command line parameter. Please refer to your platform documentation for further information.

  • If rte_cpu_get_intrinsics_support() function indicates that rte_power_monitor_multi() function is supported by the platform, then monitoring multiple Ethernet Rx queues for traffic will be supported.

  • If rte_cpu_get_intrinsics_support() function indicates that only rte_power_monitor() is supported by the platform, then monitoring will be limited to a mapping of 1 core 1 queue (thus, each Rx queue will have to be monitored from a different lcore).

  • If rte_cpu_get_intrinsics_support() function indicates that neither of the two monitoring functions are supported, then monitor mode will not be supported.

  • Not all Ethernet drivers support monitoring, even if the underlying platform may support the necessary CPU instructions. Please refer to Overview of Networking Drivers for more information.

2.8.2. API Overview for Ethernet PMD Power Management

  • Queue Enable: Enable specific power scheme for certain queue/port/core.

  • Queue Disable: Disable power scheme for certain queue/port/core.

  • Get Emptypoll Max: Get the configured number of empty polls to wait before entering sleep state.

  • Set Emptypoll Max: Set the number of empty polls to wait before entering sleep state.

  • Get Pause Duration: Get the configured duration (microseconds) to be used in the Pause callback.

  • Set Pause Duration: Set the duration of the pause (microseconds) used in the Pause mode callback.

  • Get Scaling Min Freq: Get the configured minimum frequency (kHz) to be used in Frequency Scaling mode.

  • Set Scaling Min Freq: Set the minimum frequency (kHz) to be used in Frequency Scaling mode.

  • Get Scaling Max Freq: Get the configured maximum frequency (kHz) to be used in Frequency Scaling mode.

  • Set Scaling Max Freq: Set the maximum frequency (kHz) to be used in Frequency Scaling mode.

2.9. Uncore API

2.9.1. Abstract

Uncore is a term used by Intel to describe the functions of a microprocessor that are not in the core, but which must be closely connected to the core to achieve high performance: L3 cache, on-die memory controller, etc. Significant power savings can be achieved by reducing the uncore frequency to its lowest value.

2.9.2. Intel Uncore

The Linux kernel provides the driver “intel-uncore-frequency” to control the uncore frequency limits for x86 platform. The driver is available from kernel version 5.6 and above. Also CONFIG_INTEL_UNCORE_FREQ_CONTROL will need to be enabled in the kernel, which was added in 5.6. This manipulates the context of MSR 0x620, which sets min/max of the uncore for the SKU.

2.9.3. AMD EPYC Uncore

On AMD EPYC platforms, the Host System Management Port (HSMP) kernel module facilitates user-level access to HSMP mailboxes, which are implemented by the firmware in the System Management Unit (SMU). The AMD HSMP driver is available starting from kernel version 5.18. Please ensure that CONFIG_AMD_HSMP is enabled in your kernel configuration.

Additionally, the EPYC System Management Interface In-band Library for Linux offers essential API, enabling user-space software to effectively manage system functions.

2.9.4. Uncore API Overview

Overview of each function in the Uncore API, with explanation of what they do. Each function should not be called in the fast path.

Uncore Power Init

Initialize uncore power, populate frequency array and record original min & max for die on pkg.

Uncore Power Exit

Exit uncore power, restoring original min & max for die on pkg.

Get Uncore Power Freq

Get current uncore freq index for die on pkg.

Set Uncore Power Freq

Set min & max uncore freq index for die on pkg to specified index value (min and max will be the same).

Uncore Power Max

Set min & max uncore freq to maximum frequency index for die on pkg (min and max will be the same).

Uncore Power Min

Set min & max uncore freq to minimum frequency index for die on pkg (min and max will be the same).

Get Num Freqs

Get the number of frequencies in the index array.

Get Num Pkgs

Get the number of packages (CPU’s) on the system.

Get Num Dies

Get the number of die’s on a given package.

2.10. References