SoC Architecture: NVIDIA's Denver CPU

It admittedly does a bit of a disservice to the rest of the Nexus 9 both in terms of hardware and as a complete product, but there’s really no getting around the fact that the highlight of the tablet is its NVIDIA-developed SoC. Or to be more specific, the NVIDIA-developed Denver CPUs within the SoC, and the fact that the Nexus 9 is the first product to ship with a Denver CPU.

NVIDIA for their part is no stranger to the SoC game, now having shipped 5 generations of Tegra SoCs (with more on their way). Since the beginning NVIDIA has been developing their own GPUs and then integrating those into their Tegra SoCs, using 3rd party ARM cores and other 1st party and 3rd party designs to fully flesh out Tegra. However even though NVIDIA is already designing some of their own IP, there’s still a big leap to be made from using licensed ARM cores to using your own ARM cores, and with Denver NVIDIA has become just the second company to release their own ARMv8 design for consumer SoCs.

For long time readers Denver may feel like a long time coming, and that perception is not wrong. NVIDIA announced Denver almost 4 years ago, back at CES 2011, where at the time they made a broad announcement about developing their own 64bit ARM core for use in wide range of devices, ranging from mobile to servers. A lot has happened in the SoC space since 2011, and given NVIDIA’s current situation Denver likely won’t be quite as broad a product as they first pitched it as. But as an A15 replacement for the same tablet and high performance embedded markets that the TK1-32 has found a home in, the Denver-based TK1-64 should fit right in.


K1-64 Die Shot Mock-up (NVIDIA)

Denver comes at an interesting time for NVIDIA and for the ARM SoC industry as a whole. Apple’s unexpected launch of the ARMv8 capable Cyclone core in 2013 beat competing high-performance ARMv8 designs by nearly a year. And overall Apple set a very high bar for performance and power efficiency that is not easily matched and has greatly impacted the development and deployment schedules of other ARMv8 SoCs. At the same time because Cyclone and its derivatives are limited to iOS devices, the high-performance Android market is currently served by a mix of ARMv7 designs (A15, Krait, etc) and the just recently arrived A57 and Denver CPUs.

Showcasing the full scope of the ARM architecture license and how many different designs can execute the same instruction set, none of these ARMv8 CPUs are all that much alike. Thanks to its wide and conservatively clocked design, Apple’s Cyclone ends up looking a lot like what a recent Intel Core processor would look like if it were executing ARM instead of x86. Meanwhile ARM’s A57 design is (for lack of a better term) very ARMy, following ARM’s own power efficient design traditions and further iterating on ARM’s big.LITTLE philosophy to pair up high performance A57 and moderate performance A53 cores to allow a SoC to cover a wide power/performance curve. And finally we have Denver, perhaps the most interesting and certainly least conventional design, forgoing the established norms of Out of Order Execution (OoOE) in favor of a very wide in-order design backed by an ambitious binary translation and optimization scheme.

Counting Cores: Why Denver?

To understand Denver it’s best to start with the state of the ARM device market, and NVIDIA’s goals in designing their own CPU core. In the ARM SoC space, much has been made of core counts, both as a marketing vehicle and of value to overall performance. Much like the PC space a decade prior, when multi-core processors became viable they were of an almost immediate benefit. Even if individual applications couldn’t yet make use of multiple cores, having a second core meant that applications and OSes were no longer time-sharing a single core, which came with its own performance benefits. The OS could do its work in the background without interrupting applications as much, and greedy applications didn’t need to fight with the OS or other applications for basic resources.

However also like the PC space, the benefits of additional cores began to taper off with each additional core. One could still benefit from 4 cores over 2 cores, but unless software was capable of putting 3-4 cores to very good use, generally one would find that performance didn’t scale well with the cores. Compounding matters in the mobile ecosystem, the vast majority of devices run apps in a “monolithic” fashion with only one app active and interacting with the user at any given point in time. This meant that in absence of apps that could use 3-4 cores, there weren’t nearly as many situations in which multitasking could be employed to find work for the additional cores. The end result has been that it has been difficult for mobile devices to consistently saturate an SoC with more than a couple of cores.

Meanwhile the Cortex family of designs coming from ARM have generally allowed high core counts. Cortex-A7 is absolutely tiny, and even the more comparable Cortex-A15 isn’t all that big on the 28nm process. Quad core A15 designs quickly came along, setting the stage for the high core count situations we previously discussed.

This brings us to NVIDIA’s goals with Denver. In part due to the issues feeding 4 cores, NVIDIA has opted for a greater focus on single-threaded performance than the ARM Cortex designs they used previously. Believing that fewer, faster cores will deliver better real-world performance and better power consumption, NVIDIA set out to build a bigger, wider CPU that would do just that. The result of this project was what NVIDIA awkwardly calls their first “super core,” Denver.

Though NVIDIA wouldn’t know it at the time it was announced in 2011, Denver in 2015 is in good company that helps to prove that NVIDIA was right to focus on single-threaded performance over additional cores. Apple’s Cyclone designs have followed a very similar philosophy and the SoCs utilizing them remain the SoCs to beat, delivering chart-topping performance even with only 2 or 3 CPU cores. Deliver something similar in performance to Cyclone in the Android market and prove the performance and power benefits of 2 larger cores over 4 weaker cores, and NVIDIA would be well set in the high-end SoC marketplace.

Performance considerations aside, for NVIDIA there are additional benefits to rolling their own CPU core. First and foremost is that it reduces their royalty rate to ARM; ARM still gets a cut as part of their ISA license, but that cut is less than if you are also using ARM licensed cores. The catch of course is that NVIDIA needs to sell enough SoCs in the long run to pay for the substantial costs of developing a CPU, which means that along with the usual technical risks, there are some financial risks as well for developing your own CPU.

The second benefit to NVIDIA then is differentiation in a crowded SoC market. The SoC market has continued to shed players over the years, with players such as Texas Instruments and ST-Ericsson getting squeezed out of the market. With so many vendors using the same Cortex CPU designs, from a performance perspective their SoCs are similarly replaceable, making the risk of being the next TI all the greater. Developing your own CPU is not without risks as well – especially if it ends up underperforming the competition – but played right it means being able to offer a product with a unique feature that helps the SoC stand out from the crowd.

Finally, at the time NVIDIA announced Denver, NVIDIA also had plans to use Denver to break into the server space. With their Tesla HPC products traditionally paired x86 CPUs, NVIDIA could never have complete control over the platform, or the greater share of revenue that would entail. Denver in turn would allow NVIDIA to offer their own CPU, capturing that market and being able to play off of the synergy of providing both the CPU and GPU. Since then however the OpenPOWER consortium happened, opening up IBM’s POWER CPU lineup to companies such as NVIDIA and allowing them to add features such as NVLink to POWER CPUs. In light of that, while NVIDIA has never officially written off Denver’s server ambitions, it seems likely that POWER has supplanted Denver as NVIDIA’s server CPU of choice.

Introduction Designing Denver
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  • mkygod - Saturday, February 7, 2015 - link

    I think so to. The 3:2 ratio is one of the things that Microsoft has gotten right with their Surface Pro devices. It's the perfect compromise IMO Reply
  • UtilityMax - Sunday, February 8, 2015 - link

    I am a little perplexed by this comment. A typical user will be on the web 90% of time. Not only the web browser does not need to be natively designed or optimized for any screen ratio, but it also will be more usable on a 4:3 screen. So will the productivity apps. The only disappointment for me on the 4:3 screen would be with watching the widescreen videos or TV shows. Moreover, there is quite a bit of evidence than a lot of the next generation tablets will be 4:3. Samsung's next flagship tablet supposedly will be 4:3. Reply
  • gtrenchev - Wednesday, February 4, 2015 - link

    Anandtech is becoming more and more boring last year. Sparse on reviews, short on tech comments, lacking on depth and enthusiasm. I can see Anandtech has become a just job for you guys, not the passion it was for Anand :-) And yes, his absence is definitely noticeable.

    George
    Reply
  • Ian Cutress - Wednesday, February 4, 2015 - link

    Was the Denver deep-dive not sufficient enough? Always welcome for comments.
    As for timing, see Ryan's comment above.
    We've actually had a very good quarter content wise, with a full review on the front page at least four out of every five weekdays if not every weekday.
    Reply
  • milkod2001 - Wednesday, February 4, 2015 - link

    Why not to post on your forum some sort of suggestion box/poll where all could say what should get reviewed first so some folks won't cry where is the review of their favorite toy :) ? Reply
  • Impulses - Wednesday, February 4, 2015 - link

    Because they'll still cry regardless, and they can't possibly work entirely based on readers' whim, doesn't make sense logistically or nor editorially... Readers might vote on five things ahead of the rest which all fall on the same writer's lap, they won't all get reviewed before the rest, or readers might not be privy to new hardware because of NDAs or cases where Anandtech can't source something for review. Reply
  • tuxRoller - Thursday, February 5, 2015 - link

    While I enjoyed the review, I would've loved to have seen the kind of code driven analysis that was done with Swift.
    In particular, how long does it take for dco to kick in. What is the IPC for code that NEVER gets optimized, and conversely, what is the IPC for embarrassingly instruction-wise parallel code? Since it's relying on ram to store the uops, how long does the code need to run before it breaks even with the arm decoder? Etc.
    Reply
  • victorson - Wednesday, February 4, 2015 - link

    Are you guys kidding? Better late than never, but heck.. this is freaking late. Reply
  • abufrejoval - Wednesday, February 4, 2015 - link

    Thanks for making it worth the wait!

    The in-depth analysis of Denver is uniquely Anandtech, because you can't get that anywhere else.

    And while Charly D. is very entertaining, the paywall is a bit of an impediment and I quite like again the Anand touch of trying to be as fair as possible.

    I was and remain a bit worried that there seems to be no other platform for Denver, which typically signals a deeper flaw with an SoC in the tablet and phone space.

    While I'm somewhat less worried now, that Denver might be acceptable as a SoC, the current Nexus generation is no longer attractive at these prices, even less with the way the €/$ is evolving.
    Reply
  • Taneli - Wednesday, February 4, 2015 - link

    eDRAM cache à la Crystalwell would be interesting in a future Denver chip. Reply

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