45. 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.

45.1. 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.

45.2. 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.

45.3. 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.

45.4. 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.

45.5. 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.

45.6. User Cases

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

45.7. Empty Poll API

45.7.1. Abstract

For packet processing workloads such as DPDK polling is continuous. This means CPU cores always show 100% busy independent of how much work those cores are doing. It is critical to accurately determine how busy a core is hugely important for the following reasons:

  • No indication of overload conditions
  • User does not know how much real load is on a system, resulting in wasted energy as no power management is utilized

Compared to the original l3fwd-power design, instead of going to sleep after detecting an empty poll, the new mechanism just lowers the core frequency. As a result, the application does not stop polling the device, which leads to improved handling of bursts of traffic.

When the system become busy, the empty poll mechanism can also increase the core frequency (including turbo) to do best effort for intensive traffic. This gives us more flexible and balanced traffic awareness over the standard l3fwd-power application.

45.7.2. Proposed Solution

The proposed solution focuses on how many times empty polls are executed. The less the number of empty polls, means current core is busy with processing workload, therefore, the higher frequency is needed. The high empty poll number indicates the current core not doing any real work therefore, we can lower the frequency to safe power.

In the current implementation, each core has 1 empty-poll counter which assume 1 core is dedicated to 1 queue. This will need to be expanded in the future to support multiple queues per core.

45.7.2.1. Power state definition:

  • LOW: Not currently used, reserved for future use.
  • MED: the frequency is used to process modest traffic workload.
  • HIGH: the frequency is used to process busy traffic workload.

45.7.2.2. There are two phases to establish the power management system:

  • Training phase. This phase is used to measure the optimal frequency change thresholds for a given system. The thresholds will differ from system to system due to differences in processor micro-architecture, cache and device configurations. In this phase, the user must ensure that no traffic can enter the system so that counts can be measured for empty polls at low, medium and high frequencies. Each frequency is measured for two seconds. Once the training phase is complete, the threshold numbers are displayed, and normal mode resumes, and traffic can be allowed into the system. These threshold number can be used on the command line when starting the application in normal mode to avoid re-training every time.
  • Normal phase. Every 10ms the run-time counters are compared to the supplied threshold values, and the decision will be made whether to move to a different power state (by adjusting the frequency).

45.7.3. API Overview for Empty Poll Power Management

  • State Init: initialize the power management system.
  • State Free: free the resource hold by power management system.
  • Update Empty Poll Counter: update the empty poll counter.
  • Update Valid Poll Counter: update the valid poll counter.
  • Set the Frequency Index: update the power state/frequency mapping.
  • Detect empty poll state change: empty poll state change detection algorithm then take action.

45.8. User Cases

The mechanism can applied to any device which is based on polling. e.g. NIC, FPGA.