Save Power, Hibernate Your Embedded Linux System

This is a guest post by Tharma Rajan G, Project Lead, e-con Sytems.

What is the best way to save power consumption of your embedded Linux system? Is there any way to save max power and resume operation ? Yes. It is ‘hibernate’ mode, one of the Power Modes in Linux. This article talks about how we utilized this ‘hibernate mode’ in our Reference Platform Kit Meissa-I with eSOMiMX6-micro SOM.

Meissa-I is a dual board solution that features eSOMiMX6-micro Computer on Module & carrier board. Meissa-I development board runs Linux and Android Marshmallow (under development). eSOMiMX6-micro is based on Dual/Quad core ARM CortexTM-A9 based CPU @ 800MHz/Core. It has 1GB LPDDR2 and 4GB eMMC FLASH (expandable upto 32GB). The eSOMiMX6-micro module also has the Wireless LAN and Bluetooth module.

Linux Power Modes

Power Management is a key feature in embedded Linux system and there are two types for implementing the power management on x86 platforms:

1. Advanced Power Management (APM)
2. Advanced Configuration and Power Interface (ACPI)

ACPI is the newer of the two technologies and puts power management in the hands of the operating system, allowing for more intelligent power management than is possible with BIOS controlled APM.

For ARM based systems however, APM/ACPI is not used. Instead power management in ARM Linux System, is implemented using sysfs entries. Following are sysfs entries for power management.

The power management subsystem provides a unified sysfs interface to userspace, regardless of what architecture or platform one is running. Generally sysfs filesystem is mounted in /sys directory and one can see that power management related entries in /sys/power path.

Using the entry /sys/power/state, one can get supported power states (or power modes) of the embedded system. And also one can put that embedded system in one of those supported power states.

System Sleeping States and Description

Swap Memory for Hibernate Power Mode

Swap memory

Swap memory is the area on disk that is reserved to be used as extra RAM when your system needs more RAM than what is available. When your embedded Linux system runs out of free memory, then kernel can move some of the inactive pages into swap partition to make free room for active pages in RAM memory. If you plan to use hibernation in your embedded Linux systems, then you swap space have to be at least twice the RAM capacity.

Kernel support for swap memory

Enable swap support in Linux kernel

Test swap memory

1. To test swap memory, we created a simple test application test_swap_mem which will allocate memory infinitely using malloc() API and fill that allocated memory using memset() API. Let’s have a look at the source code
   
2. Before creating swap space, Check whether swap space is enabled using free command
   Here swap total , used and free space is marked as 0 which means swap space in not enabled yet.
3. Run test_swap_mem application as discussed below
   
   Let us wait till the application reaches OOM.

   

   From the above image, you can observe test_swap_mem allocates maximum of 799MB, after that it reaches OOM and also you can confirm swap space is not used by observing Free swap and Total swap is marked as 0 kB and no swap pages are used

4. Now let we create a swap memory region and test with the same application. Once you decided about the partition block to be used as swap space , you have to mark that partition block as swap partition.
   
5. After marking that specific partition as swap partition , you have to prepare it using mkswap tool
   
6. Now your swap space is ready to use , then activate swap partition.
   
7. First notice how much swap memory is available and then run test_swap_mem application.
   Here 892892 bytes of swap memory region is available. Try to explore how much swap space is used and when OOM will be reached.

   

   Yes , swap memory is utilized. As there is no free memory (including swap) available for allocation, kernel throws OOM error hence kills that test application.

Hibernate – Power Mode

Hibernation power mode saves the system state (i.e all the pages created in RAM memory) to swap space and put the system power down.When the system is powered on, the state is restored (i.e pages are moved from swap to RAM memory). To write the system state to swap space, a mechanism called ‘swsusp’ (Swap Suspend ) is used.

Kernel Configuration for Hibernate mode

Now we will prepare the Swap Space to hold the Hibernation Image.

Setup the swap area,

Enable partition for swapping

Run a process, to verify restoring the hibernation image.

Entering into Hibernate

1. Enter into hibernation state by executing below command.
   

Resume from Hibernate

Before starting to resume from hibernate state, append resume=/pathto/swapspace command line parameter to read back hibernate image from that specific swap partition. Let us change the bootargs for hibernate resume state , and run the bootcmd.

Now you will observe, hibernate image is read from swap partition and resumed.

Analysis

Power Measurement Analysis

The power data is obtained by computing the product of voltage and current measured.

Hardware and Software Used:

1. The software version used for the measurement is
   • U-Boot 2016.03
   • Linux Kernel Version 4.1.15.
   • Yocto Krogoth X11 image.
2. The board used for the measurement is Meissa-I with eSOMiMX6-micro SOM.
3. The measurements were performed using Fluke 15B+ Digital Multimeter.

Power Consumption details in tested power modes.

System Power Consumption Chart

Active Mode Power Consumption

Standby Mode Power Consumption:

Suspend Mode Power Consumption:

Hibernate mode Power Consumption:

Domain Power Consumption Chart :

Boot Time Analysis

App launch time Measurement in Normal Boot.

We have taken our eSOMiMX6Micro SOM as reference, it takes nearly 19 secs to execute watch command which is part of rc.local script file.

App launch time measurement in Resume Mode.

While resuming from hibernate, it takes 16.76 secs to retain and execute the same watch command process state.

Normal Boot and Hibernation Resume Comparison Chart.

Conclusion

From the above analysis on ‘hibernate mode’, we find it to be the best choice for putting the system in low power state. Also, it consumes minimal time for ‘hibernate resuming’ as compared to normal booting time. The only downside we observed is that, manual interaction was required to resume the device from hibernate mode.
I believe, this hibernate power mode analysis will be very useful to you. We will see you next time with another interesting article.

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