Processor Subject To Change

Customizable processors leverage the best of power management; a variety of approaches combine energy efficiency without sacrificing performance.

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By Ann Steffora Mutschler
With power complexity driving sophisticated management techniques, SoC design engineering teams are turning to a new class of customizable processor architectures from ARM, CEVA, NVIDIA, Qualcomm and Tensilica and others to take advantage of the best in power saving techniques.

While these new architectures are novel approaches, the concepts are not especially new, particularly in mobile applications.

“If you look at what mobile processors have been doing, I would argue they’ve been doing some sort of big.LITTLE for a long time,” explained Nandan Nayampally, director of applications processor marketing in the processor division of ARM. “By that I mean you have microcontrollers taking charge when the big application processor is not working, or you’ve got video engines being separate from the main application processing. The compartmentalization of the activities around the chip have been always a focus for mobile because you will save power any which way you can. That’s a given.”

ARM has observed that what’s changed in the recent past is that the main OS needs to be running more and more of the time because with apps like Twitter feeds and Facebook updates, those are little apps that are constantly running on top of the OS.

As fun and/or useful as they are, these apps are killing battery life.

Nayampally explained the big.LITTLE architecture with an example. “Let’s say I’m doing an MP3 playback in the old days. You’d say, ‘I’m running on the big core, I kick off the task to a little core and then turn off the big processor because the MP3 can run just fine on a microcontroller type device. It’s all on the same die. Then suddenly you get a call and it wakes up the big processor and it takes over again. But when you offloaded that MP3 in the olden days—six months or so ago—you actually could have a separate task that wasn’t really run by the OS. Now there are so many more things and services that people are coming to expect that you can’t have them done specifically for targets that are different from the application processor itself and they run on top of the OS. Now you are telling the chip, ‘No, I won’t do these specialized things as separate things for very power-efficient sub-components, they have to be done by the main processor.’ But the main processor also has to become very schizophrenic in the level of performance it requires for the main tasks as well as what it needs for the little tasks.”

Source: ARM

What makes big.LITTLE interesting is that the processors are fully coherent so the software engineer doesn’t have to worry as much about maintaining every piece of data. The coherency in hardware takes care of that. That makes the software development quicker and can actually improve performance and battery life.

Designed to be an extension of DVFS, there are multiple use models in which big.LITTLE can work, with the simplest use meant to be effectively transparent to the OS, Nayampally continued. “The power management software always speaks to a driver that is the right power and performance needed based on what is required. If, for example, you had today’s processor and it was using the lowest performance level it could while doing Twitter update, it just can’t be as efficient as something that was designed to be a fifth smaller or something like that. What if your DVFS had a next step that is more efficient and you can work there for a while? From an OS standpoint, or an application standpoint, it doesn’t matter. It’s just another step in your DVFS. Underneath it what happens is the driver now can do the kick-off to switch the operations from the big core to the little core or from the little core to the big core or cluster in fact.”

NVIDIA’s Tegra 3 employs variable symmetric multiprocessing (vSMP) while Qualcomm uses asynchronous symmetrical multiprocessing (aSMP) – which are the same principles that govern ARM’s big.LITTLE architecture.

NVIDIA’s Tegra 3, launched last November is a quad-core mobile processor for smartphones and tablets, currently shipping in the ASUS Transformer Android tablet. A company spokesman explained that behind Tegra 3’s power efficiency is a fifth lower-power “companion” CPU core that goes with the four CPU cores and is specifically targeted at battery savings. Tegra 3’s architecture allows it to provide the best combination of performance and battery life by switching between the four main CPU cores and the fifth core for less demanding tasks and active standby mode.

For CEVA, which licenses DSPs, programmability has always been the name of the game, according to Eran Briman, the company’s vice president of marketing. About seven years ago it became apparent that general-purpose DSPs are not going to make the cut for next-generation designs—particularly in 40nm communications designs. In one of its newest offerings, the CEVA-XC DSP software-defined radio architecture, users can run the complete receive and transmit channels entirely in software, except for very few hardware engines that simply don’t make sense in software, he said. To accompany this and to allow for advanced power management, CEVA recently released a software development kit that includes advanced power management. Looking ahead, Briman believes there will be fully programmable communications units on SoCs.

CEVA isn’t the only company in the DSP space to see this trend.

“Many baseband designs particularly, when they are operating on complex protocols and care a lot about energy have moved to neither completely hard-wired—because that would be too fragile or intolerant of inevitable corrections and improvements—nor completely general-purpose, because a general-purpose processor is generally much less energy-efficient than something that is more specific to the task at hand,” observed Chris Rowen, CTO at Tensilica. “Especially in low-power baseband processing, we’re seeing more and more optimization of programmable engines to do this, where the baseband subsystem might include 6 or 8 or 10 different cores that are programmable. Some of these still may be fairly general-purpose, because you may say in this function though there’s a wide variety of different tasks that I need to do on the data and it is more energy efficient for me to have one that is shared among these different, diverse functions than to have one piece of hardware for every single function. That would make it too big. Having a programmable solution can in some cases also make it a smaller solution. In general, small is good for energy.”

Tensilica offers a range of DSP cores. It also allows users to build their own customized dataplane processors.



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