Power Management - The Ohio State University

Power Management - The Ohio State University
Power Management
Feng Qin
CSE Dept., The Ohio State University
• Linux Power Management
• Android Power Management
• Ongoing Research Project
Linux Power Management
• APM (Advanced Power Management)
– Controlled by BIOS (need reboot for reconfiguration)
– Activated when system becomes idle
• E.g., screen saver => sleep => suspend
– OS has no knowledge about when the system
change power states
• ACPI (Advanced Configuration and Power
Interfaces) Í Newer
• First released in 1996 by Compaq/HP,
Intel, Microsoft, Phoenix and Toshiba
• ACPI is an interface specification
comprised of both software and hardware
• Controlled by OS
– During OS initialization, OSPM takes over the
power management functions
(Source: ACPI Specification)
ACPI Functions (I)
• System power management
– the entire computer
• Device power management
– ACPI tables describe motherboard devices,
their power states, the power planes the
devices are connected to
• Processor power management
– When OS is idle but not sleeping, it puts
processors in low-power states
ACPI Functions (II)
• Device and processor performance
– When system is active, OSPM balances the
performance and energy conservation
• Configuration / Plug and Play
– Information used to enumerate and configure
motherboard devices
• System Events
– A general event mechanisms (thermal events,
power management events, docking, etc.)
ACPI Functions (III)
• Battery management
– Move policy from APM BIOS to ACPI OS
– Require a Smart Battery subsystem interface
• Thermal management
– Allow OEMs to define thermal zones, thermal
indicators, and cooling methods
• Embedded controller
• SMBus controller
Global System Power States
(Source: ACPI Specification)
Linux System Power
Management States
• Three possible system states
– Standby / Power-On Suspend
– Suspend-to-RAM
– Suspend-to-Disk
Standby / Power-On Suspend
• ACPI State: S1
• String: “standby”
• Offer minimal power saving, with lowlatency transition back to working
• Put devices to a low-power state D1
– If not support D1, left them on
• Transition from standby to on takes about
1-2 seconds
ACPI State: S3
String: “mem”
Offer significant power savings
System and device states are saved and
kept in memory
• All devices are suspended and put into D3
• Memory is placed in self-refresh mode to
retain its contents
• Resume takes about 3-5 seconds
ACPI State: S4
String: “disk”
Offer the greatest power savings
Similar to STA, but add an extra step to
write memory contents to disk
• Can be handled by the firmware (stored in
swap space) or the kernel
Power Management Interface
• Linux provides a unified sysfs interface to
• In /sys/power directory (mounting sysfs in
/sys first)
• /sys/power/state controls system power
– Read the supported states
– Write to the file cause state transition
Runtime Power Management
• Runtime PM (Android uses it to some
• Hardware components may be powered
down when they are not used and powered
up when they are needed again
– Display dimmed or turned off
– Reduce capacity of various system
components if full capacity is not needed
Android Power Management
• Wakelocks or suspend blockers
– Aggressive utilization of full system suspend
to save as much energy as reasonably possible
– Nature state of the system is a sleep state
• Energy is only used for refreshing memory and
providing power to a few devices that can generate
signals to wake the system up
– Working state is only entered in response to a
wakeup signal from one of the select devices
• When the work is done, the system goes to sleep
• Two types of wakelocks:
• When a “IDLE” wakelock is held, the
system will not enter idle (low power) state
• When a “SUSPEND” wakelock is held, the
system will not enter suspend state
• Currently defined wakelocks are listed at
Manipulating a Wakelock
• Kernel space
– wake_lock_init (struct wake_lock *lock, int
type, const char *name);
– wake_lock(struct wake_lock *lock);
– wake_unlock(struct wake_lock *lock);
• User space
– A process write a name to
– A process write the lock name to
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