Balancing Performance and Power Consumption
Once 98-pound weaklings compared to their desktop siblings, mobile processors — CPUs designed for notebook PCs, where power, cooling, and physical space are all at a premium — have steadily narrowed the performance gap over time. And while mobile chips command higher prices (just as LCD screens do over CRT monitors), the premium is often worthwhile when size, weight, and battery life are important concerns.
“The gap between desktop and mobile CPUs has shrunk quite a bit over the years,” says Masa Okumura, director of product marketing and worldwide product planning at Toshiba America Information Services in Irvine, Calif. “But desktops will always have an advantage because they have bigger chassis and better heat dissipation, which allows you to increase the clock speed of a desktop CPU sooner than a mobile CPU.”
At this writing, for instance, AMD’s fastest notebook chip is its mobile Athlon XP 1800+ versus the desktop Athlon XP 2600+. Intel’s fastest desktop Pentium 4 runs at 2.8GHz, while what it calls the Mobile Pentium 4 Processor-M currently tops out at 2.2GHz.
And while the performance difference has narrowed, the price difference is still considerable — AMD charges PC vendors virtually the same price for the mobile Athlon XP 1800+ and desktop 2600+, while Intel’s 2.53GHz desktop P4 is cheaper than the 2.0GHz mobile part. “That makes perfect sense,” insists Mike Stinson, vice president of mobile products for Gateway in San Diego, Calif.: “In order to push the performance up, it gets harder and harder to do what you have to do to make a product a mobile product.”
Throttling Down: SpeedStep and PowerNow
One way that mobile processors reduce their appetite for power is by changing their clock speed and voltage based on user or application demands — speeding up or slowing the CPU when there’s more or less for it to do. Intel implements this concept with its SpeedStep scheme, which first appeared in the company’s mobile Pentium III in January 2000.
“SpeedStep drops the frequency at which a CPU is operating and the voltage used to operate it,” says Okumura. “[In its simplest form, when] a laptop is attached to a power outlet, its CPU will operate at top speed, but when it’s running off its battery, it will keep the CPU’s speed down.” Intel’s current notebook CPUs use what the company calls Enhanced SpeedStep technology to permit dynamic switching between full and partial speed even during battery work sessions.
Since June 2000, AMD’s mobile processors have offered a power-management feature called PowerNow. Like SpeedStep, PowerNow adjusts a CPU’s operating frequency and voltage based on processing demand. But where SpeedStep offers only a high and low gear, so to speak — the 1.4GHz through 2.0GHz mobile Pentium 4s all slow to 1.2GHz when SpeedStep kicks in — PowerNow offers a third mode that automatically detects the performance required by a software application and adjusts clock speed for the best compromise between horsepower and battery life.
According to AMD, the technology can provide very fine control over a CPU’s voltage and frequency, supporting up to 32 different core-voltage settings (from 0.925V to 2.0V) and speed increments of 33MHz or 50MHz from a bottom of 133MHz or 200MHz all the way to a CPU’s maximum clock rate. The bottom line? AMD claims up to 30 percent longer battery life without a noticeable difference in performance or responsiveness for the laptop user.
More Power To You
Assisting SpeedStep and PowerNow in their power-juggling act is a technology called ACPI: Advanced Configuration and Power Interface, an open standard developed by Compaq, Microsoft, Phoenix, and Toshiba. Expanding on an older protocol dubbed APM (Advanced Power Management), this specification provides BIOS-based power management and uses device activity timeouts to determine when transitions should be made into lower-power states.
“This interface helps facilitate the flow of information about how much power is being used by a system’s components and makes that information available at the BIOS and operating system level,” explains Okumura. “That allows the system to look at appropriate components and shut things down.”
According to Okumura, Windows 98 was the first version of the operating system to take advantage of ACPI, but it wasn’t until the release of Windows XP that the technology hit its stride. “The tricky part about ACPI is that is connected to the applications sitting on top of Windows,” he says. “When Windows 98 was launched, there were no ACPI-compliant applications. Now, with the release of Office XP, everything is ACPI-compliant.”
Moreover, you can control some power-hungry devices from Windows’ Power Options Control Panel. In Win XP, for example, you can choose from a number of power schemes such as Home/Office Desk, Portable/Laptop, Presentation, and Max Battery, as well as specifying how long after its last use to switch off a system’s display and hard disk and when to enter sleep and hibernation states.
The latter are early laptop battery-saving inventions still used today, although far less sophisticated than dynamic adjustments like PowerNow or SpeedStep. Sleep or “suspend” mode shuts off power to all of a PC’s components save its memory, so when “awakened” it can return within seconds to its pre-sleep state, with operating system, applications, and data files picking up where they left off.
The drawback to sleep mode is that even the trickle required to sustain system memory eventually drains the battery, so work in progress can be lost. By contrast, “hibernation” writes the current contents of memory to the hard disk before turning off all power, so wakeup is more like performing a cold boot, except the system is booting from the pre-sleep information stored on the hard drive.
Extreme Energy Savings
While Intel and AMD have turned their R&D; muscle to making mobile processors with desktop oomph, gigahertz-plus performance has taken a back seat to energy efficiency and thermal concerns at Santa Clara, Calif.-based Transmeta Corp., maker of the Crusoe TM5800 — a CPU used mostly in ultralight, slimline notebooks seen mostly in the Japanese market, but found stateside in everything from NEC’s nearly silent (fanless) desktop, the PowerMate eco, to prototypes of Microsoft’s forthcoming Tablet PC.
One way Crusoe manages heat buildup is by having fewer heat-generating transistors than its more conventional competitors. It does this by using software to perform some of the functions handled by hardware in other processors. This “Code Morphing” software, which is loaded into memory at system bootup (before the BIOS and operating system), lets the chip run cooler by removing tasks that tax it the hardest — determining which instructions to execute and when to execute them. Transmeta says the scheme cuts processor power consumption by 60 to 70 percent and doubles the life of a battery charge: “Whenever we can move functionality from hardware into the software, we’re saving power,” notes vice president for marketing and chief technology officer David Ditzel.
Crusoe also counters SpeedStep and PowerNow with LongRun, which monitors CPU activity and manages power consumption based on demand. “We can change the megahertz and voltage of our chip hundreds of times per second — so fast that we can change it between every frame of a DVD movie,” Ditzel says. “When you go to a more complex frame, we can use a little more megahertz; when you go to a simpler frame, we can use less megahertz.”
Ditzel asserts that LongRun works better than rival schemes because the latter work through the operating system to change CPU voltage and core speed, whereas the Code Morphing software layer works beneath the OS and is invisible to it, but responds almost instantaneously to changes in processor demand.
Nevertheless, there’s only so much any CPU can do to lengthen a laptop’s battery life — the system’s backlit display usually consumes even more power than the processor, with its hard disk and CD or DVD drive close behind, and portable makers are now having to accommodate another battery-sucker: “Something being added to notebooks now that is very power-hungry is wireless,” Stinson notes. “A lot of people using [802.11b wireless networking] now like thin, light notebooks. That’s not necessarily a good fit.”
One reason why WiFi consumes so much power, Stinson explains, is that the radio used for connecting to the network must be always on; it can’t be powered down like other components unless the user is sure he or she won’t need a network link.
Indeed, one of this year’s biggest trends has been a surge in the number of portable computers built around desktop rather than mobile processors. Some reasons are the smaller die sizes and relatively cool-running, power-frugal characteristics of newer 0.13- rather than 0.18-micron-process chips: Since most Pentium 4 “Northwood” desktop CPUs draw 1.5V, down from 1.75V for the original desktop P4 “Willamette,” many vendors have opted to settle for that rather than pay the premium for the 1.3V mobile Pentium 4. (Intel offers low- and ultra-low-power, Crusoe-competitor chips, but they belong to the older Pentium III family, at least until successors codenamed “Banias” ship in early 2003.)
Other reasons involve a portable PC’s target audience. Systems built around mobile chips appeal to frequent fliers and other users who most value mobility. Desktop-processor-based laptops are desktop-replacement systems, whose users are more likely to move from one wall outlet to another than from airplane to hotel. These users are willing to choose a heavier system with briefer battery life in exchange for desktop amenities such as 15- or 16-inch displays and huge hard drives.
“In the last year or so, we’ve seen an emergence of a market segment that is more portable than mobile,” explains Stinson. “A good example of that is college students — they may move [only] every six to nine months. During their work time, they may spend a couple of hours in their dorm room and a couple of hours at the library, but wherever they are, they’re almost always at a plug. Battery life isn’t important. Screen size is more important than weight. And they want as close to desktop performance as they can get.”
Still, while desktop CPUs may always outperform their roving relatives, options like the mobile Athlon XP 1800+ and 1.9GHz mobile Pentium 4 make performance an increasingly relative term. “If you’re looking for performance for your daily business tasks,” Okumura says, “you don’t necessarily need the latest, fastest processor. The performance of mobile CPUs is more than adequate for 80 to 85 percent of applications today.”