Optimizing Power Efficiency and Performance of Embedded Linux Systems

A comprehensive guide on understanding and implementing power management techniques for embedded Linux systems

Power management is a critical aspect of embedded Linux systems as it can significantly impact the device's power consumption and battery life. Through this article, we shall explore and provide an in-depth understanding of the power management features available in embedded Linux systems and techniques to achieve power efficiency.

Embedded Linux systems have several built-in power management features that can be used to optimise power efficiency. Some of the most commonly used features include

CPU frequency scaling is a technique used to adjust the clock speed of a CPU based on its workload. The main goal of this technique is to reduce power consumption by lowering the CPU frequency when the workload is low and increasing the CPU frequency when the workload is high. The kernel controls the CPU frequency scaling through the cpufreq (CPU Frequency Scaling) subsystem. This subsystem provides an interface for userspace (such as cpufreq-set) to set the CPU frequency and governor and for programs to get the current CPU frequency.

Power-saving governors are algorithms that control how the CPU frequency is scaled based on the workload. The Linux kernel provides several governors, including:

• OnDemand: This governor increases the CPU frequency when the workload is high and decreases it when it is low.
• Performance: This governor sets the CPU frequency to the maximum value at all times, which can be helpful in performance-critical tasks but will consume more power.
• powersave: This governor sets the CPU frequency to the minimum value at all times, which can be helpful for power-saving but will decrease performance.

Users can switch between governors to select the one best fits their needs. Some governors are available only for certain types of CPU; for example, some governors are only available for intel-pstate drivers.

By default, most of the embedded Linux systems use the OnDemand governor, which balances performance and power consumption well. But it can be changed based on the specific requirements of the system.

It's important to note that while CPU frequency scaling can help reduce power consumption, it can also impact the system's performance. Therefore, it's essential to carefully monitor the system's performance when changing the CPU frequency scaling settings.

The Power Management Interface (PMI) is a kernel-level interface that can configure power management settings on embedded Linux systems. It is implemented as a set of files in the /sys/power/ directory, which can be read and written using standard Linux file operations.

The PMI provides a way for userspace to configure power management settings, such as the CPU frequency scaling governor, the idle state of the CPU, and the system sleep state. It also provides a way to read the current power management settings, such as the current CPU frequency and the current sleep state of the system.

For example, to configure the CPU frequency scaling governor, you can write the desired governor to the /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor file. To configure the idle state of the CPU, you can write the desired idle state to the /sys/devices/system/cpu/cpu0/cpuidle/stateX/disable file, where X is the idle state number.

Idle states are low-power states that the CPU can enter when idle. The number of idle states and their implementation will vary depending on the specific CPU and system. Each idle state has a different power consumption level and latency. The deeper the idle state, the lower the power consumption, but the higher the latency to wake up from that state. Governors can be used to configure the minimum and maximum idle state the CPU should use, the threshold at which the CPU should transition between idle states and the latency of each idle state.

By using the PMI interface and the idle states, developers can configure the system to enter the lowest possible idle state when the CPU is idle, which can help to reduce power consumption and prolong the battery life of embedded Linux systems.

Kernel configuration

The Linux kernel is the core of an embedded Linux system. Optimising the Linux kernel for power efficiency can dramatically impact an embedded Linux system's power consumption and battery life. Building and configuring a custom kernel tailored to the system's requirements can also help optimise power efficiency. The custom kernel could be built by enabling and configuring power-saving features such as:

• The tickless kernel is a feature that reduces the number of timer interrupts that the kernel generates, which can help to reduce power consumption. To enable the tickless kernel, the kernel configuration option "CONFIG_NO_HZ" should be set. This option can be set using the menuconfig or config tool during the kernel build process.
• Power-aware scheduling is a feature that allows the kernel to consider power consumption when scheduling processes. The kernel provides several power-aware scheduling policies such as "SCHED_POWERSAVE" and "SCHED_BATCH". These policies can be set using the sched_setscheduler() system call.
• Runtime Power Management (RPM) is a feature that allows the kernel to dynamically adjust the power state of devices based on their usage. The kernel configuration option "CONFIG_PM_RUNTIME" should be set to enable RPM. Once enabled, devices can be placed into low-power states when not in use to save power.

It's important to note that the availability and functionality of these features may vary depending on the system and the version of the kernel.

Optimising Applications for Power Efficiency

The power consumption of a system is not only determined by the kernel and the hardware but also by the applications running on the system. Optimising embedded Linux applications for power efficiency can significantly impact power consumption and battery life. Here is a list of considerations for reducing power consumption in embedded Linux applications.

• Reducing CPU Usage: One of the most effective ways to reduce power consumption in embedded Linux applications is to reduce CPU usage. This can be achieved by using efficient algorithms, minimising the number of instructions executed, and reducing the number of context switches.
• Minimising Memory Usage can also help to reduce power consumption in embedded Linux applications. This can be achieved by using memory-efficient data structures, reducing the number of memory allocations, and using memory compression techniques.
• Reducing I/O activity can also help to reduce power consumption in embedded Linux applications. This can be achieved by reducing the number of I/O operations, using buffered I/O, and using asynchronous I/O.
• Optimising GUI Applications: To maximise power efficiency in GUI applications, developers can use techniques such as reducing the number of updates to the screen, using efficient drawing algorithms and minimising the use of animations.
• Optimising Networking Applications: To maximise power efficiency in networking applications, developers can use techniques such as reducing the number of network connections, reducing the amount of data transferred, and using power-efficient communication protocols.
• Optimising Multimedia Applications: To maximise power efficiency in multimedia applications, developers can use techniques such as reducing the resolution and frame rate of video streams, using efficient codecs, and minimizing the use of hardware acceleration.

Measuring and Monitoring Power Consumption

Measuring and monitoring power consumption is an essential step in optimizing the power efficiency of an embedded Linux system. Several command-line tools can measure and monitor power consumption on embedded Linux systems. Some of the most commonly used tools include powertop and iotop.

• Powertop is a Linux utility that can measure and monitor power consumption on embedded Linux systems. It provides detailed information on the power consumption of different system components, such as the CPU, memory, and I/O devices. It also provides suggestions on how to reduce power consumption.
• Iotop is a Linux utility that can measure and monitor I/O activity on embedded Linux systems. It provides detailed information on the I/O activity of different processes, including the amount of data read and written, the number of I/O operations, and the I/O bandwidth.

Measuring Power Consumption Using Hardware-based Tools

In addition to command-line tools, hardware-based power measurement tools, such as power meters and oscilloscopes, can also measure and monitor power consumption on embedded Linux systems.

• Power meters are hardware devices that can be used to measure the power consumption of an embedded Linux system. They can provide detailed information on the power consumption of different system components, such as the CPU, memory, and I/O devices.
• Oscilloscopes are hardware devices that can be used to measure and analyze the power consumption of an embedded Linux system. They can provide detailed information on the power consumption of different system components, such as the CPU, memory, and I/O devices.

conclusion

As Embedded Linux adaption across various verticles of devices continues to rise, power efficiency is still a significant concern. By understanding the power management features and implementing the appropriate controls, Developers can take steps to improve the power efficiency of Linux devices. This article helps developers formulate proper strategies and policies for achieving a more suitable balance of performance and power efficiency.