Smartphone CPUs put desktops to shame

The latest smartphones sport multicore processors of dizzying complexity

Fighting severe size and power constraints, the makers of smartphones have achieved levels of ingenuity not seen on the desktop. This results in mobile devices that not only have multiple cores, but multiple sizes and types of cores.

For instance, phone component maker Qualcomm's flagship Snapdragon 820 system on a chip (SoC) for mobile devices has two types of central processor, explains Cisco Cheng, Qualcomm's manager of technical marketing. Plus, it has graphics, camera, sensor, location, peripheral, Wi-Fi, Bluetooth, signal, wireless modem, and memory processors or controllers, each able to handle its task more efficiently than the CPU could. (In November, Qualcomm announced its successor, the Snapdragon 835, adding high-speed charging to the mix.)

The resulting class of increasingly powerful mobile technology will almost certainly start showing up on the desktop -- and perhaps even converge with desktop systems.

The mobile landscape

"Multi-core smartphones got off to a later start than multi-core desktops but caught up quickly," says Joshua Ho, an editor at the hardware review and analysis site AnandTech. "It took a few years for desktops to move from single-core to dual cores, but on the mobile side it only took six months to a year, starting with the Samsung Galaxy S II in 2011."

But while the desktop is dominated by the x86 processor dynasty that originated at Intel, mobile devices predominately contain ARM processors designed by UK-based ARM Holdings. (ARM originally stood for Advanced RISC Machines. It was acquired by the Tokyo-based multinational Softbank Group in September 2016.)

"The mobile world standardized on ARM processors the same way the desktop world standardized on Intel x86 processors," says independent microcomputer consultant Jim Turley. "The reasons mostly have to do with marketing and chance, and -- in ARM's case -- politics. ARM is a UK firm and the first successful mobile phone makers were Finnish and Swedish, so there was a certain charm in choosing a European vendor. Also, ARM was happy to license the chip design so it could be customized to fit small spaces, and it had a reputation for low power consumption."

Another big difference from the desktop world is that ARM does not make its processors, but licenses its designs to component and phone makers. The devices are often made in third-party fabs.

There are a number of ways to license the ARM processor, and that license is a major component of the licensee's business model, explains Patrick Moorhead, head of the analyst firm Moor Insights & Strategy. "At the top ARM will license the instruction set. The only two in the mobile world to do that are Qualcomm and Apple. They build their own architecture and processor with the ARM instruction set," he explains. Doing that to create a custom core costs about $25 million and a year of effort, he estimates.

"If you have enough volume, it is less expensive [per unit] and gives you a competitive edge, but there is a higher degree of [market] risk," Moorhead says. It can take a year to get to market, and if the market changes in the meantime you will suffer.

On the next rung are those who license the generic design of a specific core. ARM will work with the licensee to get the design through the fab and onto the system chip, and will guarantee the results. Doing so costs less up front but, due to higher royalties and fees, will in the end cost twice as much per unit, Moorhead says.

Literature supplied by ARM Holdings shows the firm actually has at least seven levels of licensing options. The two least expensive are for academics and experimenters and do not involve commercialization. The next four are commercial licenses involving increasing interaction with ARM, increasing amounts of knowledge access, higher licensing fees and investments, and fewer restraints on the licensee. ARM refers to the top (seventh) level, which involves licensing the instruction set, as the architecture level.

The literature shows that in the fall of 2016 ARM had 1,396 revenue-generating license holders, of whom 20 were at the architecture level.

"Most flagship [smartphone] products over $600 use a customized ARM processor, while the lower-end products use generic ARM processors," says Ben Bajarin, an analyst at the market research firm Creative Strategies. "Qualcomm and Apple have deep technical differences between each other, but both take the time to customize their architecture, and end up with something more powerful and with more capabilities" than the generic smartphones, he said.

big.LITTLE for multi-core

The basic approach ARM has used for multi-core architecture, which it styles big.LITTLE, involves having two kinds of cores, typically called big and little. The big ones are intended to run foreground tasks at maximum speed, and the little ones are intended to run background tasks at maximum power efficiency.

"If you look at multi-core x86 processors for the PC, all the cores are the same," explains James Bruce, lead mobile strategist for ARM Holdings. "ARM decided to do something different and came up with the concept of big.LITTLE. They are compatible cores that share the same resources, but one group is implemented for efficiency and the other for performance. This lets you tune your CPU to meet the needs of the smartphone workload," he says.

Bruce says that the larger cores have sophisticated features like branch prediction and out-of-order execution, while the smaller ones are serial processors with fewer logic gates, which consequently use less power. Both run the same machine code, he adds. For eight cores a standard configuration would be four big and four little, Bruce says.

"By implementing the same CPU two ways, you have the best of both worlds," he adds.

But there are many ways to implement big.LITTLE. Qualcomm, Cheng explains, uses two grades of processor on the Snapdragon 820 that are identical except that they have different circuit geometries and run at different speeds. The slower ones, of course, use less power.

Apple would not comment on its ARM implementation, but is famous for sticking to a dual-core design. "Apple dual-core designs often outperform eight-core designs," says Creative Strategies' Bajarin. "It all comes down to the quality of the design. They tune their user experience with a customized GPU and their own software."

But many smartphone vendors prefer multiple cores if only because they are a marketing feature, says Linley Gwennap, analyst at The Linley Group. "Another thing we have seen is that eight is a very lucky number in China and so eight-core phones are popular due to that association, while there might not be much technical benefit."

Multi-core benefit?

Indeed, since phone use is not the same as desktop use, some sources question the need for ever more cores in smartphones.

"Two to three cores right now is the sweet spot in phone applications," Moor Insights' Moorhead says. "There are some workloads, like uploading apps and using the phone at the same time, that can benefit from more cores." Most apps take advantage of one, fewer leverage two, and rare cases, like updating all apps at the same time, use three. Some games use three, and fancier games might need four.

"I think it'll be a very long time before we can make the case that we actually need eight processor cores," Moorhead says. "Background tasks and multitasking are not the world of smartphones."

"We have reached the number of cores that are useful, which is three to four when you have only one type of core, or double that with big.LITTLE, not counting other processors like the GPU," AnandTech's Ho explains. "That is where growth will be -- adding GPUs, vector processors, video decoders, and fixed function blocks that give more performance or power efficiency," he says.

That will mean increasing emphasis on adding heterogeneous devices to SoCs, and the Heterogeneous Systems Architecture Foundation has been set up to facilitate the task. John Glossner, foundation president, says the challenge is to have all the devices share the same memory space. Long-term, the foundation's open-source standards will lead to programming languages that handle parallelization automatically, he adds.

The future

ARM Holdings' Bruce notes that benchmark scores for smartphones have become comparable to those of typical laptops, so that a convergence of the two worlds looks inevitable.

And indeed, sources point to the HP Elite X3 and the Microsoft Lumia 950 XL. Both are Windows 10 devices that can plug into larger displays and keyboards. Once plugged in, they can run the full-featured versions of Microsoft applications thanks to the Microsoft Continuum feature, Qualcomm's Cheng explains.

"Convergence is real -- it's only a matter of time," Cheng predicts.

But that's for office users. For average users who talk on their phones and do some surfing and video-watching, the increasing power of the devices has little meaning, Cheng agrees. Part of this failing is due to the app makers, he says.

"Developers rarely exert themselves to multi-thread an app, and they are not going to write an app that won't run on a $150 phone," -- i.e., one that is not top of the line -- says Cheng. For average users, the benefit of improved technology is enhanced power efficiency, which leads to better battery life with existing use cases, he says.

However, the expected advent of 5G wireless connectivity will extend fiber-like speeds to mobile devices, opening new horizons, he predicts. That could allow multi-person teleconferencing, telepresence apps, and drastically decreased latency, he says. It would presumably lead to new uses, such as cloud integration for apps, promoting desktop convergence, he adds.

As for cores, "I don't think we'll get much past eight anytime soon," says the Linley Group's Gwennap. "We've been doing eight for a couple of years now, and the industry seems to be stuck. It gets back to the software -- we're not seeing software that can use eight cores, so why go to 16?"

"There will be more heterogeneous processing with lopsided mixes of more types of processors," predicts consultant Turley. "There will be more voice recognition so you don't have to type on the screen."

"But something that radically changes how we see CPU design is unlikely," Ho adds.

This story, "Smartphone CPUs put desktops to shame" was originally published by Computerworld.

Copyright © 2017 IDG Communications, Inc.

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