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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  • seanleeforever - Wednesday, February 4, 2015 - link

    2nd that.
    I am not here to read about how fast the tablet is or how nice it looks. i am here for in depth content about the chip. would it be nice that this content was available since the release of the product? absolutely, but given the resource it would either be a brief review that is going to be the same as review you can find from hundred of other websites, or late but in depth.
    honestly i think anand should be targeting at more tech oriented contents that's few but in depth, and leave the quick/dirty review for other websites.

    superb job.
  • WaitingForNehalem - Wednesday, February 4, 2015 - link

    Yeah but who cares about tablets??!! I don't come to Anandtech to read about budget tablets, or SFF PCs, or more smartphones. The Denver coverage was not even that in depth TBH, just commentary on the NVidia slides. I have a EE degree and some of the previous write ups were so in depth they could be class material. This one isn't which is fine but I don't think it excuses how late it came out. The enthusiast market is growing and you should be targeting that demographic as you previously have, not catering to the mainstream like hundreds of sites already do.
  • retrospooty - Wednesday, February 4, 2015 - link

    The enthusiast market is growing ? What with CPU's not really getting, or needing to be any faster for several years now, and a standard mid range quad core i5 (non-overclocked) being WAY more than powerful enough to run 99.9% of anything out there, how is the enthusiast market is growing? Most enthusiasts I know don't even bother any more... There just isnt a need. Any basic PC is great these days.
  • WaitingForNehalem - Wednesday, February 4, 2015 - link

    I totally agree with you. That doesn't change the fact that the market is growing as more users are adopting gaming PCs. Enthusiasts now actually command a sizable portion of desktops sold. Intel's Devil's Canyon was in response to that.
  • retrospooty - Thursday, February 5, 2015 - link

    OK, I get what you mean.

    I guess I am still in a mind set where a PC "enthusiast" is your overclocker, tweaker, buying the latest and fastest of everything to eek out that extra few frames per second.

    Today, a mid range quad core i5 from 3 years ago and a decent mid-high range card runs any game quite nicely.
  • FunBunny2 - Thursday, February 5, 2015 - link

    There was a time, readers may be too young to have been there, when there was a Wintel monopoly: M$ needed faster chips to run ever more bloated Windoze and Intel needed a cycle-sink to soak up the increase in cycles that evolving chips provided. Now, we're near (or at?) the limits of single-threaded performance, and still haven't found a way to use multi-processor/core chips in individual applications. There just aren't a) many embarrassingly parallel problems and b) algorithms to turn single-threaded problems into parallel code. I mean, the big deal these days is 4K displays? It looks prettier, to some eyes, but doesn't change the functionality of an application (medical and such excepted, possibly).

    Does anyone really need an i7 to surf the innterTubes for neater porn?
  • nico_mach - Friday, February 6, 2015 - link

    I think the chip coverage was superb, I don't have an EE degree and I'm pretty sure that's what the website is steered towards. And I still think I got it.

    It's fascinating the number of layers involved in this Android tablet, and speaks to why Apple can optimize so much better. There's the chip->NVIDIA chip optimizer->executable code->Dalvik compiler/runtime->dalvik code. I mean, when the lags are encountered, that's twice as many suspects to investigate.

    I still think that the review is a little harsh on Denver. It's hitting the right performance envelope at the right price. While it's an mildly inefficient design, clearly NVIDIA is pricing it accordingly, and that might be a function of moving some of the optimization work to software. And that's work that Apple and MS do all the time - Apple much more successfully, obviously. There's a real gap in knowledge of how efficient Apple's chips are vs how optimized the software/hardware pairing is.
  • dakishimesan - Wednesday, February 4, 2015 - link

    I have no interest in tablets, but the deep dive on Denver was a fascinating read, and still completely relevant even if the product is a few months old. Thanks for the great review.
  • Sindarin - Wednesday, February 4, 2015 - link

    ...can I offer you a cup of hot chicken soup laddy? .....maybe some vicks vapor rub? lol! c'mon dude! we're all sick(vaca) in December!
  • hahmed330 - Wednesday, February 4, 2015 - link

    Hi, outstanding article with incredible attention to detail... Do you think its possible to run Dynamic Code Optimizer on per say 2 or maybe even 4 small cpu cores dedicated to doing all the software OoOE functions instead of using time slicing? (A53s or just some XYZ narrow cores for a potential 2+2 or 4+4 or maybe even 8+8)

    Also whats the die size of a denver core in comparison to a enhanced cyclone core?? That is where a lot of gains are possible potentially 30%-50%..

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