fa77f80f49
In the References section in the Power Management overview, both links pointed to the same l3fwd-power app. Fix the links so that one points to l3fwd-power, and the other points to the vm_power_manager sample app. Signed-off-by: David Hunt <david.hunt@intel.com> Acked-by: Marko Kovacevic <marko.kovacevic@intel.com>
203 lines
7.3 KiB
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203 lines
7.3 KiB
ReStructuredText
.. SPDX-License-Identifier: BSD-3-Clause
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Copyright(c) 2010-2014 Intel Corporation.
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Power Management
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================
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The DPDK Power Management feature allows users space applications to save power
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by dynamically adjusting CPU frequency or entering into different C-States.
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* Adjusting the CPU frequency dynamically according to the utilization of RX queue.
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* Entering into different deeper C-States according to the adaptive algorithms to speculate
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brief periods of time suspending the application if no packets are received.
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The interfaces for adjusting the operating CPU frequency are in the power management library.
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C-State control is implemented in applications according to the different use cases.
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CPU Frequency Scaling
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---------------------
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The Linux kernel provides a cpufreq module for CPU frequency scaling for each lcore.
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For example, for cpuX, /sys/devices/system/cpu/cpuX/cpufreq/ has the following sys files for frequency scaling:
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* affected_cpus
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* bios_limit
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* cpuinfo_cur_freq
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* cpuinfo_max_freq
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* cpuinfo_min_freq
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* cpuinfo_transition_latency
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* related_cpus
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* scaling_available_frequencies
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* scaling_available_governors
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* scaling_cur_freq
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* scaling_driver
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* scaling_governor
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* scaling_max_freq
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* scaling_min_freq
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* scaling_setspeed
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In the DPDK, scaling_governor is configured in user space.
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Then, a user space application can prompt the kernel by writing scaling_setspeed to adjust the CPU frequency
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according to the strategies defined by the user space application.
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Core-load Throttling through C-States
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-------------------------------------
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Core state can be altered by speculative sleeps whenever the specified lcore has nothing to do.
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In the DPDK, if no packet is received after polling,
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speculative sleeps can be triggered according the strategies defined by the user space application.
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Per-core Turbo Boost
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--------------------
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Individual cores can be allowed to enter a Turbo Boost state on a per-core
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basis. This is achieved by enabling Turbo Boost Technology in the BIOS, then
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looping through the relevant cores and enabling/disabling Turbo Boost on each
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core.
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Use of Power Library in a Hyper-Threaded Environment
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----------------------------------------------------
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In the case where the power library is in use on a system with Hyper-Threading enabled,
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the frequency on the physical core is set to the highest frequency of the Hyper-Thread siblings.
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So even though an application may request a scale down, the core frequency will
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remain at the highest frequency until all Hyper-Threads on that core request a scale down.
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API Overview of the Power Library
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---------------------------------
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The main methods exported by power library are for CPU frequency scaling and include the following:
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* **Freq up**: Prompt the kernel to scale up the frequency of the specific lcore.
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* **Freq down**: Prompt the kernel to scale down the frequency of the specific lcore.
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* **Freq max**: Prompt the kernel to scale up the frequency of the specific lcore to the maximum.
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* **Freq min**: Prompt the kernel to scale down the frequency of the specific lcore to the minimum.
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* **Get available freqs**: Read the available frequencies of the specific lcore from the sys file.
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* **Freq get**: Get the current frequency of the specific lcore.
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* **Freq set**: Prompt the kernel to set the frequency for the specific lcore.
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* **Enable turbo**: Prompt the kernel to enable Turbo Boost for the specific lcore.
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* **Disable turbo**: Prompt the kernel to disable Turbo Boost for the specific lcore.
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User Cases
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----------
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The power management mechanism is used to save power when performing L3 forwarding.
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Empty Poll API
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--------------
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Abstract
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~~~~~~~~
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For packet processing workloads such as DPDK polling is continuous.
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This means CPU cores always show 100% busy independent of how much work
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those cores are doing. It is critical to accurately determine how busy
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a core is hugely important for the following reasons:
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* No indication of overload conditions
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* User does not know how much real load is on a system, resulting
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in wasted energy as no power management is utilized
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Compared to the original l3fwd-power design, instead of going to sleep
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after detecting an empty poll, the new mechanism just lowers the core frequency.
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As a result, the application does not stop polling the device, which leads
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to improved handling of bursts of traffic.
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When the system become busy, the empty poll mechanism can also increase the core
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frequency (including turbo) to do best effort for intensive traffic. This gives
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us more flexible and balanced traffic awareness over the standard l3fwd-power
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application.
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Proposed Solution
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~~~~~~~~~~~~~~~~~
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The proposed solution focuses on how many times empty polls are executed.
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The less the number of empty polls, means current core is busy with processing
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workload, therefore, the higher frequency is needed. The high empty poll number
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indicates the current core not doing any real work therefore, we can lower the
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frequency to safe power.
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In the current implementation, each core has 1 empty-poll counter which assume
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1 core is dedicated to 1 queue. This will need to be expanded in the future to
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support multiple queues per core.
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Power state definition:
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^^^^^^^^^^^^^^^^^^^^^^^
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* LOW: Not currently used, reserved for future use.
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* MED: the frequency is used to process modest traffic workload.
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* HIGH: the frequency is used to process busy traffic workload.
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There are two phases to establish the power management system:
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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* Training phase. This phase is used to measure the optimal frequency
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change thresholds for a given system. The thresholds will differ from
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system to system due to differences in processor micro-architecture,
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cache and device configurations.
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In this phase, the user must ensure that no traffic can enter the
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system so that counts can be measured for empty polls at low, medium
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and high frequencies. Each frequency is measured for two seconds.
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Once the training phase is complete, the threshold numbers are
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displayed, and normal mode resumes, and traffic can be allowed into
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the system. These threshold number can be used on the command line
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when starting the application in normal mode to avoid re-training
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every time.
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* Normal phase. Every 10ms the run-time counters are compared
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to the supplied threshold values, and the decision will be made
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whether to move to a different power state (by adjusting the
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frequency).
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API Overview for Empty Poll Power Management
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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* **State Init**: initialize the power management system.
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* **State Free**: free the resource hold by power management system.
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* **Update Empty Poll Counter**: update the empty poll counter.
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* **Update Valid Poll Counter**: update the valid poll counter.
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* **Set the Frequency Index**: update the power state/frequency mapping.
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* **Detect empty poll state change**: empty poll state change detection algorithm then take action.
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User Cases
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----------
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The mechanism can applied to any device which is based on polling. e.g. NIC, FPGA.
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References
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----------
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* The :doc:`../sample_app_ug/l3_forward_power_man`
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chapter in the :doc:`../sample_app_ug/index` section.
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* The :doc:`../sample_app_ug/vm_power_management`
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chapter in the :doc:`../sample_app_ug/index` section.
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