At the end of January, UC Berkeley EECS Professor Jan Rabaey gave a comprehensive one-evening course in low-power design essentials to about 100 people attending a meeting of the Santa Clara Valley chapter of the IEEE Solid State Circuits Society. It was a comprehensive presentation, given the amount of time available, and it extended information in Rabay’s book, Low Power Design Essentials, published by Springer in 2009. In this blog post and subsequent posts, I will attempt to summarize more than two hours of Rabaey’s rapid-fire presentation.
“Why is power important today?” That’s how Rabaey started his presentation. He answered his question this way:
“Power now plays a role in virtually every component” of the electronic ecosystem, which consists of an infrastructural core—the “cloud” built with a massive number of computer racks, servers, high-speed routers, storage systems, and cooling systems; multiple networks of mobile devices connected to the cloud through cellular, WiFi, and other wireless and wired networks; and then the extended ecosystem of all electrically powered devices, which Rabaey called the “sensor swarm.” There are millions of servers and other large systems in the cloud—a constantly growing number. There are already billions of mobile devices connected to the cloud, and there are or will be hundreds of billions or trillions of devices in the sensor swarm. Each of these ecosystem niches has very different needs with respect to power and energy.
The majority of all computation is headed to the cloud. Why? That’s where the largest and most concentrated energy is located. That’s where the cooling systems are located. That’s where there’s room for massive amounts of storage. Ultimately, said Rabaey, 99% of all computation will be performed in the cloud because our appetite for computation and storage is insatiable. We will feed that ferocious appetite mostly by growing the cloud and mobile devices will be our means of accessing information in the cloud.
The mobile device tier exists to put information into and to extract information from the cloud. We can already see that tier expanding rapidly with Smartphones and tablets currently taking the lead. No doubt we will invent more device types as we devise new ways to collect and interpret the information we’re storing in the cloud.
Rabaey foresees an immense sensor swarm. He believes that sensors will essentially become as numerous as grains of sand on the beach. They’ll be in our walls, on our bodies, and in our bodies. Eventually, there will be trillions of these sensors in the swarm, all reporting to the cloud, so that the entire ecosystem can become more aware of its physical surroundings and manage itself accordingly. A simple example: a room should be able to switch its lights off when there’s no one in the room.
Each of these three ecosystem tiers has different power and energy needs. In fact, power (used as a synonym for both power and energy from this point on in this blog entry) plays a critical role for each of the three ecosystem tiers and it’s one of the most compelling factors driving the design of devices and systems in each tier. Performance is still key, but power is equally important because if you cannot design a device or system to work within the available power envelope, then you will not get the desired performance.
Rabaey then turned to the central tier—the cloud. After one or two years, he said, the annual cost of running a data center is approximately equal to the cost of powering and cooling the center. In other words, once the capital equipment is in place, equipment maintenance and upgrades are a small fraction of ongoing the operating cost. The energy costs needed to run and cool the data center consume almost the entire annual operating budget of the center and those costs are large—millions of dollars per center.
Mobile phone handsets have a different sort of power constraint: about 3W. That’s about all you can get from today’s battery technology and about all you can dissipate in a person’s hand without burning the skin. Asbestos gloves are not likely to become “hot” fashion accessories.
As far as new, more advanced battery technology, Rabaey said “Battery’s Law is slow, way slower than Moore’s Law.” However, the market is not so patient as to put up with Battery’s Law. Performance requirements are always escalating so design engineers must scale component power consumption to get more performance and more function from the same amount of power.
We have started to turn to multicore designs to get more performance per Watt, said Rabaey, “but multicore platforms are only a partial answer.” The real opportunities, he said, will come from architectural innovations—they will come from a system perspective.
Let me repeat that, because it’s been said for 10 years and it’s not sinking in based on my own observations:
The real opportunities will come from a system perspective.
Because, as Rabaey next said, once you’ve tried all the circuit tricks in the book (and we have, as you’ll see), you have to ask yourself “What can I do next?”
The sensor swarm, I think, is where Rabaey’s real interest lies. Why? Because I’ve seen him give presentations about sensor networks for years. Rabaey called sensors “disappearing electronics.” They need to be very low in cost, very small, and they need to be self-contained from an energy perspective.
Now mobile devices also need to be self-contained, but we often recharge them or change their batteries. That will not work for the sensor swarm. “We can’t replace batteries in trillions of devices,” said Rabaey. In other words, Rabaey believes that these sensors must either carry all the energy they’ll ever need in the form of a battery that neither needs replacing or charging or these sensor systems must be able to harvest useful energy from their environment (light, heat, electromagnetic radiation, or vibration). Such systems must use mere microwatts of power. This is a very tall order for today’s designers.
After refining the definition of the three ecosystem tiers, Rabaey then differentiated the power and energy needs of the three tiers. Power is more important for high-performance systems in the cloud. The central issue is heat removal followed by the delivery of peak power and then energy cost. Portable systems are all about battery life while “zero-power” sensors have unique requirements for energy scavenging and storage.
Having set the stage by defining the three electronic ecosystem tiers, Rabaey then gave a review of existing low-power design techniques that focused on digital circuit power and energy consumption. Although circuit power and energy consumption consists of two parts (dynamic and static), designers were able to ignore static power consumption (leakage) during most of the CMOS age, which really started in the early 1980s. During this golden age, process lithography scaling was a win-win proposition because it delivered linear power scaling with improved circuit speed. Who couldn’t love that? In fact, we loved it so much, we gave other power-reduction design techniques either lip service or a lick and a prayer. Circuit scaling worked so well, who could ask for anything more?
That was the golden age and this is now.
As we continued to scale into the “deep submicron” and then nanometer regions, we also began to reduce the power supply voltages to further cut power consumption. Old-timers will remember a time when everything digital (well, nearly everything digital) ran from a 5V power supply rail. This supply voltage was a holdover from the bipolar TTL (transistor-transistor logic) days of the 1970s and 1980s and that power supply voltage level stayed with us for a very long time. (Note: Really old timers will remember that the earlier RTL (resistor-transistor logic, NOT “register transfer level”) integrated circuits ran on 3V or 3.6V, so 5V is not a magic supply voltage. It just seemed that way for more than two decades.)
Eventually, however, we needed to continue reducing power consumption and dropping the power supply voltage was a great, relatively pain-free way to cut power consumption levels. So we first dropped supply levels to 3.3V.
Wow! Instant power reduction of nearly 60%!
Then came 2.5V quickly followed by a sliding decline to supply rails near 1V. If you want, you can actually run digital nanometer CMOS circuitry at 400 or 500mV.
But there was a trap waiting at lower supply voltage levels and we’ve sprung it. As the power supply voltage starts to approach the threshold voltage of the digital transistors, transistors don’t switch as fast. Once we got to 90nm, CMOS circuits started to lose performance even though transistor sizes continued to shrink under the unblinking glare of Moore’s Law. We’d upheld Moore’s Law but we’d killed Dennard Scaling in the process.
Memo to circuit designers: loss of performance is unacceptable. It’s uncompetitive. It’s unthinkable.
So the circuit designers’ answer, of course, was to drop the transistor threshold voltage to give back some of the lost speed. The downside of this approach is that the transistors don’t turn off as fast. They stay on for longer and longer amounts of time. During that time, power flows through the partially on transistors. Nevertheless, we have taken this path with the result that leakage levels have risen to the point that just about half of the power consumed by a device is now static dissipation.
Leakage. In the words of the children’s song: “There’s a hole in the bucket dear Liza, dear Liza.”
And so scaling alone will no longer take us in the direction we want to go with respect to power consumption. Going forward from here, we will still need to use all of the circuit tricks we have developed during the golden age of CMOS but we will also need to do something—many things—more.
Fortunately, there’s a lot more we can do. I’ll discuss that topic in the next blog entry about Jan Rabaey’s remarkable short course in low-power design.