- 1 Hardware Issues
- 2 Operating System Issues
- 3 Degraded Operation
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5.9.2 Operating System Issues
The operating system plays a key role in energy management. It controls all the devices, so it must decide what to shut down and when to shut it down. If it shuts down a device and that device is needed again quickly, there may be an annoying delay while it is restarted. On the other hand, if it waits too long to shut down a device, energy is wasted for nothing.
The trick is to find algorithms and heuristics that let the operating system make good decisions about what to shut down and when. The trouble is that ''good'' is highly subjective. One user may find it acceptable that after 30 seconds of not using the computer it takes 2 seconds for it to respond to a keystroke. Another user may swear a blue streak under the same conditions. In the absence of audio input, the computer cannot tell these users apart.
The Display
Let us now look at the big spenders of the energy budget to see what can be done about each one. The biggest item in everyone's energy budget is the display. To get a bright sharp image, the screen must be backlit and that takes substantial energy. Many operating systems attempt to save energy here by shutting down the display when there has been no activity for some number of minutes. Often the user can decide what the shutdown interval is, pushing the trade-off between frequent blanking of the screen and using the battery up quickly back to the user (who probably really does not want it). Turning off the display is a sleep state because it can be regenerated (from the video RAM) almost instantaneously when any key is struck or the pointing device is moved.
One possible improvement was proposed by Flinn and Satyanarayanan (1999). They suggested having the display consist of some number of zones that can be independently powered up or down. In Fig. 5-2, we depict 16 zones using dashed lines to separate them. When the cursor is in window 2, as shown in Fig. 5-2(a), only the four zones in the lower righthand corner have to be lit up. The other 12 can be dark, saving 3/4 of the screen power.
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When the user moves the cursor to window 1, the zones for window 2 can be darkened and the zones behind window 1 can be turned on. However, because window 1 straddles 9 zones, more power is needed. If the window manager can sense of what is happening, it can automatically move window 1 to fit into four zones, with a kind of snap-to-zone action, as shown in Fig. 5-2(b). To achieve this reduction from 9/16 of full power to 4/16 of full power, the window manager has to understand power management or be capable of accepting instructions from some other piece of the system that does. Even more sophisticated would be the ability to partially illuminate a window that was not completely full (e.g., a window containing short lines of text could be kept dark on the right hand side).
Figure 5-2 The use of zones for backlighting the display. (a) When window 2 is selected it is not moved. (b) When window 1 is selected, it moves to reduce the number of zones illuminated.
The Hard Disk
Another major villain is the hard disk. It takes substantial energy to keep it spinning at high speed, even if there are no accesses. Many computers, especially laptops, spin the disk down after a certain number of minutes of activity. When it is next needed, it is spun up again. Unfortunately, a stopped disk is hibernating rather than sleeping because it takes quite a few seconds to spin it up again, which causes noticeable delays for the user.
In addition, restarting the disk consumes considerable extra energy. As a consequence, every disk has a characteristic time, Td, that is its break-even point, often in the range 5 to 15 sec. Suppose that the next disk access is expected to
some time t in the future. If t < Td, it takes less energy to keep the disk spinning
rather than spin it down and then spin it up so quickly. If t > Td, the energy saved makes it worth spinning the disk down and up again much later. If a good prediction could be made (e.g., based on past access patterns), the operating system could make good shutdown predictions and save energy. In practice, most systems are conservative and only spin down the disk after a few minutes of inactivity.
Another way to save disk energy is to have a substantial disk cache in RAM. If a needed block is in the cache, an idle disk does not have to be restarted to satisfy the read. Similarly, if a write to the disk can be buffered in the cache, a stopped disk does not have to restarted just to handle the write. The disk can remain off until the cache fills up or a read miss happens.
Another way to avoid unnecessary disk starts is for the operating system to keep running programs informed about the disk state by sending it messages or signals. Some programs have discretionary writes that can be skipped or delayed. For example, a word processor may be set up to write the file being edited to disk every few minutes. If the word processor knows that the disk is off at the moment it would normally write the file out, it can delay this write until the disk is next turned on or until a certain additional time has elapsed.
The CPU
The CPU can also be managed to save energy. A laptop CPU can be put to sleep in software, reducing power usage to almost zero. The only thing it can do in this state is wake up when an interrupt occurs. Therefore, whenever the CPU goes idle, either waiting for I/O or because there is no work to do, it goes to sleep.
On many computers, there is a relationship between CPU voltage, clock cycle, and power usage. The CPU voltage can often be reduced in software, which saves energy but also reduces the clock cycle (approximately linearly). Since power consumed is proportional to the square of the voltage, cutting the voltage in half makes the CPU about half as fast but at 1/4 the power.
This property can be exploited for programs with well-defined deadlines, such as multimedia viewers that have to decompress and display a frame every 40 msec, but go idle if they do it faster. Suppose that a CPU uses x joules while running full blast for 40 msec and x/4 joules running at half speed. If a multimedia viewer can decompress and display a frame in 20 msec, the operating system can run at full power for 20 msec and then shut down for 20 msec for a total energy usage of x/2 joules. Alternatively, it can run at half power and just make the deadline, but use only x/4 joules instead. A comparison of running at full speed and full power for some time interval and at half speed and one quarter power for twice as long is shown in Fig. 5-3. In both cases the same work is done, but in Fig. 5-3(b) only half the energy is consumed doing it.
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In a similar vein, if a user is typing at 1 char/sec, but the work needed to process the character takes 100 msec, it is better for the operating system to detect the long idle periods and slow the CPU down by a factor of 10. In short, running slowly is more energy efficient than running quickly.
The Memory
Two possible options exist for saving energy with the memory. First, the cache can be flushed and then switched off. It can always be reloaded from main memory with no loss of information. The reload can be done dynamically and quickly, so turning off the cache is entering a sleep state.
Figure 5-3 (a) Running at full clock speed. (b) Cutting voltage by two cuts clock speed by two and power consumption by four.
A more drastic option is to write the contents of main memory to the disk, then switch off the main memory itself. This approach is hibernation, since virtually all power can be cut to memory at the expense of a substantial reload time, especially if the disk is off too. When the memory is cut off, the CPU either has to be shut off as well or has to execute out of ROM. If the CPU is off, the interrupt that wakes it up has to cause it to jump to code in a ROM so the memory can be reloaded before being used. Despite all the overhead, switching off the memory for long periods of time (e.g., hours) may be worth it if restarting in a few seconds is considered much more desirable than rebooting the operating system from disk, which often takes a minute or more.
Wireless Communication
Increasingly many portable computers have a wireless connection to the outside world (e.g., the Internet). The radio transmitter and receiver required are often first-class power hogs. In particular, if the radio receiver is always on in order to listen for incoming email, the battery may drain fairly quickly. On the other hand, if the radio is switched off after, say, 1 minute of being idle, incoming messages may be missed, which is clearly undesirable.
One efficient solution to this problem has been proposed by Kravets and Krishnan (1998). The heart of their solution exploits the fact that mobile computers communicate with fixed base stations that have large memories and disks and no power constraints. What they propose is to have the mobile computer send a message to the base station when it is about to turn off the radio. From that time on, the base station buffers incoming messages on its disk. When the mobile computer switches on the radio again, it tells the base station. At that point any accumulated messages can be sent to it.
Outgoing messages that are generated while the radio is off are buffered on the mobile computer. If the buffer threatens to fill up, the radio is turned on and the queue transmitted to the base station.
When should the radio be switched off? One possibility is to let the user or the application program decide. Another is turn it off after some number of seconds of idle time. When should it be switched on again? Again, the user or program could decide, or it could be switched on periodically to check for inbound traffic and transmit any queued messages. Of course, it also should be switched on when the output buffer is close to full. Various other heuristics are possible.
Thermal Management
A somewhat different, but still energy-related issue, is thermal management. Modern CPUs get extremely hot due to their high speed. Desktop machines normally have an internal electric fan to blow the hot air out of the chassis. Since reducing power consumption is usually not a driving issue with desktop machines, the fan is usually on all the time.
With laptops, the situation is different. The operating system has to monitor the temperature continuously. When it gets close to the maximum allowable temperature, the operating system has a choice. It can switch on the fan, which makes noise and consumes power. Alternatively, it can reduce power consumption by reducing the backlighting of the screen, slowing down the CPU, being more aggressive about spinning down the disk, and so on.
Some input from the user may be valuable as a guide. For example, a user could specify in advance that the noise of the fan is objectionable, so the operating system would reduce power consumption instead.
Battery Management
In ye olde days, a battery just provided current until it was drained, at which time it stopped. Not any more. Laptops use smart batteries now, which can communicate with the operating system. Upon request they can report on things like maximum voltage, current voltage, maximum charge, current charge, maximum drain rate, current drain rate, and more. Most laptop computers have programs that can be run to query and display all these parameters. Smart batteries can also be instructed to change various operational parameters under control of the operating system.
Some laptops have multiple batteries. When the operating system detects that one battery is about to go, it has to arrange for a graceful cutover to the next one, without causing any glitches during the transition. When the final battery is on its last legs, it is up to the operating system to warn the user and then cause an orderly shutdown, for example, making sure that the file system is not corrupted.
The Windows system has an elaborate mechanism for doing power management called ACPI (Advanced Configuration and Power Interface). The operating system can send any conformant driver commands asking it to report on the capabilities of its devices and their current states. This feature is especially important when combined with plug and play because just after it is booted, the operating system does not even know what devices are present, let alone their properties with respect to energy consumption or power manageability.
It can also sends commands to drivers instructing them to cut their power levels (based on the capabilities that it learned earlier, of course). There is also some traffic the other way. In particular, when a device such as a keyboard or a mouse detects activity after a period of idleness, this is a signal to the system to go back to (near) normal operation.