Low power consumption is an important goal in designing modern electronic systems. Most previous studies of low-power techniques focused on either hardware or software to reduce power consumption. “Power Management” is an effective approach to reduce power consumption by various electronic systems without any significant degrading of their performances.
What is “Power Management”?
Power management is a process that allows computers and monitors to enter low-power states when sitting idle. The low power modes of inactive computers can involve reducing central processing unit power consumption or spinning down the hard disk. Inactive monitors with enabled power management enter low-power modes by turning off monitor output.
Power management does not reduce the performance of computers and monitors. It simply adds features to reduce their power consumption at the time when they are not in use. Most power management savings come from reducing power when the machine is not fully active by adding low-power or “sleep” modes that kick in when idle. “Sleep” modes usually involve slowing the clock rate of the central processing unit, and may include spinning down the hard disk, turning off entire system components such as video cards, sound cards or disk controllers.
Why Power Management is essential?
Worldwide, computers and monitors in commercial and industrial settings use millions of kilowatts of electricity each day. Two simple steps can save more than half of this energy.
· Enable power management functions that place computers and monitors in low-power mode during periods of inactivity.
· Turning off computers and monitors after work.
When using energy efficient machines, they save money on electricity as well as reduce pollution from power plants. In addition to the environmental benefits of reducing energy consumption, power management can improve equipment reliability by reducing waste heat.
History of the Power Management
In the early days of personal computing, under operating systems like DOS and CP/M (Control Program / Monitor or Control Program for Microcomputers) operating systems, there was no such an operation called power management. Those computers either used 100% of their power requirements or were switched off. Personal computer power management history dates back at least to 1989, when Intel produced processors with technology to allow the central processing unit (CPU) to slow down, suspend, or shut down part or all of the system platform, or even the CPU itself, to preserve and extend battery life. The increasing importance of power management reflects the increasing number of PCs in use and their transition to tools that go wherever people go.
PC power management was first introduced in laptop computers to allow longer operating times while running on battery power. Later it was brought into the desktop PC market. Many early power management systems had long recovery times, awkward configuration methods and low energy savings. However, power management has improved rapidly, becoming more powerful, reliable and easier to use. It also now delivers considerably more energy savings. In 1993 Intel and Microsoft introduced Advanced Power Management (APM) which becoming an industry standard. The APM protocol supports power management by defining how power management commands are communicated within the PC system.
How does Power Management work?
As the following figure shows, computers are logically organized as a hierarchy of layers. Those at the top are the software that the user directly interacts with. Those closer to the bottom direct the physical control of electrical signals. Power management can involve the application software and the operating system (sometimes these are not involved), and always requires action by the firmware (BIOS), processor and peripheral hardware.
The BIOS (Basic Input/Output System) is a combination of hardware and firmware (software in read-only memory), distinct from the operating system, that intermediates between the processor and other parts of the system. In the first generation of power management (machines built through 1993 or 1994), it was controlled only by the BIOS. As of 1996, the BIOS is still a key component, but more of the configuration and control is rising into the operating system and occasionally into application software. However, control signals must still pass through each intermediate layer for action to occur.
Figure 1: PC Power Management key components and communication paths
In the above figure, the numbers indicate the various steps in initiating power management. It shows the communication paths which allow power management to occur.
Number 2 The BIOS send periodic signals (about once per second) to the operating system to begin power management
Number 3 If this signal is passed through by the operating system, it will trigger the start of the power management timers in the BIOS. The operating system will only pass the signal through if it detects no activity from the application software
Number 4 If there is no activity, the operating passes the signal back to the BIOS, which begins a timer
Number 1 The BIOS continues to monitor keyboard and mouse activity
Number 5 After a specified time with no activity, the BIOS will initiate power management by sending appropriate messages to some or all of the hard disk, peripheral cards, processor, and video card
After initiating a change in mode, the BIOS begins another timer which indicates when to initiate the next power management mode. If at any time the BIOS receives an interrupt request (keyboard, mouse, or network activity), the BIOS will signal the required peripheral cards, processor, and video card to return to an active mode (usually only a demand for hard disk activity will cause the hard disk to spin up). Examples of peripheral cards include network interfaces and CD-ROM drives.
Power management has four major components for the following activities.
1. To monitor activity levels of the processor, input devices (such as the keyboard and mouse), and communication peripherals (network or modem).
2. To utilize timers to decide when to initiate the shift to a lower power mode.
3. To changes the power management status need to be communicated to the correct device and actually occur.
4. To recognize when activity resumes and return to a higher power (or full-power) mode.
PC Power Management modes
Before the release of Windows 95, Microsoft operating system software was only minimally involved in desktop PC power management (and application software was only used for monitor power management, not for controlling the PC itself). Thus the BIOS was, and remains, a critical component. Three companies dominate the production of BIOS systems used in x86-based PCs. This means that computers from different manufacturers may use the same power management. The following activities applies to Advanced Power Management (APM), and not necessarily to early (1993 and earlier) implementations of power management. To enter APM modes, they must be enabled, and the specified amount of time must pass without activity. Note that hibernate is not an APM mode.
Full-on Mode:
· All components fully powered; no power management occurring.
APM Enable Mode:
· CPU is slowed or stopped (depending on BIOS); all other devices still draw full power.
· Some systems have a ‘doze’ mode that is similar to APM Enabled.
PC Savings: 0-25% Recovery time: instantaneous
APM Standby Mode:
· CPU may be stopped depending on operation or activity; most devices are in low power mode.
· Monitor enters its first power management mode.
· Activity can trigger a return to enabled or full-on, depending on the system and activity.
PC Savings: 20-30%
Monitor Savings: 60-90%
APM Suspend Mode:
· CPU is stopped; most power-managed devices are not powered (network card may stay on).
· Maximum power savings under APM.
· Activity can trigger a return to standby, doze or full-on, depending on the BIOS
PC Savings: 25-45%
Monitor Savings: 0-10% Recovery time: 3 to 10 seconds
Hard Disk Power Down : (this is not an APM mode)
· Hard disk spin in stopped; this is independent of other power management (hence not a system mode), so that the remainder of the system can be fully operational or power down.
· Disk control electronics are still powered to facilitate quick reactivation
PC Savings: 10%
Recovery time: 3–10 seconds (disk savings independent of other savings)
Hibernate: (this is not an APM mode)
· All memory contents and system state saved to disk.
· System resistant to power loss
PC Savings: 90-100% Recovery Time: 15-60 seconds
Workstation Savings: 95% Recovery Time: <60>
Off Mode
· No operational parameters are saved
· System resets and starts at full-on mode
· Most systems use no power (a few draw a small amount).
PC Power Management with Networks
Maintaining network connections during power management was a problem for many earlier systems. Some new systems continue to have problems with this, but many have been tested to work properly under common networking systems. Currently, most power management problems with networked PCs are a result of the way the network operating system works rather than with the PC hardware.
Monitor Power Management
Even though the PC must be the initiator, power management has been more successful in monitors than in PCs. Compared to power managing PCs, monitors are usually simpler, have much more energy savings potential, power manage more reliably, and are less likely to interfere with operation or network connections. Because of this, it is even more important to enable monitors for power management than it is to enable PCs.
Monitor power management is in most cases independent of PC power management in that the monitor can power down even if the PC doesn’t, and vice-versa. However, the monitor is still dependent on the PC for power management initiation. This is necessary since the monitor does not directly receive the activity information needed to know when to begin and end power management. Once the first low-power mode is entered, however, the monitor has an internal timer and will shift to succeeding low-power modes even if the PC doesn’t send additional signals. While delay times may differ, for the most part, the monitor and PC are driven by the same activity for beginning and ending low-power modes (though network activity is an exception to this).
The structure of monitor power management control is shown in Figure 1. The PC always initiates the process, with the initial timers within the BIOS, or special software that comes with the video card. Successive power management modes can be activated by either the PC's timer or the monitor's internal timer.
Some PCs have a “convenience” electrical outlet on the back for plugging in the monitor. PCs that can switch off the power to this outlet allow energy savings from monitors that can't power manage on their own. In addition, if the monitor is plugged into the PC (whether the outlet is power-managed or not), then the monitor will be switched off when the PC is switched off (otherwise they often remain on, albeit with a blank screen).
Potential Barriers to Power Management
Even though a computer or monitor may have power management features, power management may not always operate effectively. There are many reasons why power management can be defeated in systems that have the feature.
Software Interactions
Some application software can interfere with power management, depending on how it is configured and the particular machine it is used on. One example is the ‘auto-save’ feature on many word processors and spreadsheets. If the feature saves the document even if no changes have been made, this will unnecessarily cause the processor and hard disk to stay awake, defeating power management partially or entirely.
Some screensavers will periodically load complex images from the hard disk, keeping the disk from powering down. A screensaver can keep the monitor and processor from power managing, unless set to a specific power management mode; many screensavers lack a power management mode and so need to be turned off for power management to occur.
Upgrades
Power management capabilities may change when PCs are upgraded by replacing the processor, the motherboard, or add-on cards (e.g. the network interface). Software upgrades of operating systems or utility software can cause power management to be disabled. Before upgrading many similar machines, determine if the proposed change interferes with power management. If it does, consider looking for alternatives that do not.
Networks
Computer networks pose special challenges for power management. Once a PC is connected to a network, the user may want to access the machine remotely, others may rely on being able to connect to it at any time, and services such as disk backups may operate during nights or weekends. This can mean that the simplest power management strategy, simply turning the machine off, can no longer be used for the PC (though the monitor can and should be turned off). Remote access by modem has the same effect as does a computer set up to receive faxes.
Future Directions of the Power Management
Power Management in Monitors
There are several important emerging monitor technologies that offer some relief in the historical increase in power. Most of today's monitors are Cathode Ray Tubes (CRTs). The two technologies that show promise in gaining significant market share in the near future are Liquid Crystal Display technologies, and thin CRTs.
Flat-panel displays used with today's laptop computers use far less power than CRTs, but currently their high cost limits their use with desktop PCs. This is changing as manufacturers seek to bring costs down and build larger displays.
Power Management in PCs
Not only power management for PCs for periods when they are not fully active, there are also opportunities to reduce active power. Active power can be lowered with techniques such as reducing the chip count through more integration of functions, lowering the power supply voltage, using a more efficient power supply, or switching to smaller (less energy-intensive) expansion cards.
