Ever since NVIDIA arrived on the SoC scene, it has done a great job of introducing its ultra mobile SoCs. Tegra 2 and 3 were both introduced with a healthy amount of detail and the sort of collateral we expect to see from any PC silicon vendor. While the rest of the mobile space is slowly playing catchup, NVIDIA continued the trend with its Tegra 4 and Tegra 4i architecture disclosure.

Since Tegra 4i is a bit further out, much of NVIDIA’s focus for today’s disclosure focused on its flagship Tegra 4 SoC due to begin shipping in Q2 of this year along with the NVIDIA i500 baseband. At a high level you’re looking at a quad-core ARM Cortex A15 (plus fifth A15 companion core) and a 72-core GeForce GPU. To understand Tegra 4 at a lower level, we’ll dive into the individual blocks beginning, as usual with the CPU.

ARM’s Cortex A15 and Power Consumption

Tegra 4’s CPU complex sees a significant improvement over Tegra 3. Despite being an ARM architecture licensee, NVIDIA once again licensed a complete processor from ARM rather than designing its own core. I do fundamentally believe that NVIDIA will go the full custom route eventually (see: Project Denver), but that’s a goal that will take time to come to fruition.

In the case of Tegra 4, NVIDIA chose to license ARM’s Cortex A15 - the only vanilla ARM core presently offered that can deliver higher performance than a Cortex A9.

Samsung recently disclosed details about its Cortex A15 implementation compared to the Cortex A7, a similarly performing but more power efficient alternative to the A9. In its ISSCC paper on the topic Samsung noted that the Cortex A15 offered up to 3x the performance of the Cortex A7, at 4x the area and 6x the power consumption. It’s a tremendous performance advantage for sure, but it comes at a great cost to area and power consumption. The area side isn’t as important as NVIDIA has to eat that cost, but power consumption is a valid concern.

To ease fears about power consumption, NVIDIA provided the following data:

The table above is a bit confusing so let me explain. In the first row NVIDIA is showing that it has configured the Tegra 3 and 4 platforms to deliver the same SPECint_base 2000 performance. SPECint is a well respected CPU benchmark that stresses everything from the CPU core to the memory interface. The int at the end of the name implies that we’re looking at purely single threaded integer performance.

The second row shows us the SPECint per watt of the Tegra 3/4 CPU subsystem, when running at the frequencies required to deliver a SPECint score of 520. By itself this doesn’t tell us a whole lot, but we can use this data to get some actual power numbers.

At the same performance level, Tegra 4 operates at 40% lower power than Tegra 3. The comparison is unfortunately not quite apples to apples as we’re artificially limiting Tegra 4’s peak clock speed, while running Tegra 3 at its highest, most power hungry state. The clocks in question are 1.6GHz for Tegra 3 and 825MHz for Tegra 4. Running at lower clocks allows you to run at a lower voltage, which results in much lower power consumption. In other words, NVIDIA’s comparison is useful but skewed in favor of Tegra 4.

What this data does tell us however is exactly how NVIDIA plans on getting Tegra 4 into a phone: by aggressively limiting frequency. If a Cortex A15 at 825MHz delivers identical performance at a lower power compared to a 40nm Cortex A9 at 1.6GHz, it’s likely possible to deliver a marginal performance boost without breaking the power bank.

That 825MHz mark ends up being an important number, because that’s where the fifth companion Cortex A15 tops out at. I suspect that in a phone configuration NVIDIA might keep everything running on the companion core for as long as possible, which would address my fears about typical power consumption in a phone. Peak power consumption is still going to be a problem I think.

ARM's Cortex A15 Architecture
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  • tipoo - Sunday, February 24, 2013 - link

    Under 500 in Sunspider, about twice as fast as anything else ARM. But then again, it's a few months newer than that, and actually still not shipping. And as usual with Nvidia they're early to each party (first to dual core, first to quad core), but not always the best performing. We'll see if other Cortex A15 designs beat it.

    I'd love to see four of those cores paired with SGXs upcoming 600/Rogue series.
    Reply
  • jeffkibuule - Sunday, February 24, 2013 - link

    SunSpider is so software sensitive that a Tegra 3 @ 1.2 Ghz on Windows RT beats a Snapdraon S4 Pro @ 1.5Ghz on Nexus 4 using Chrome. It's a terrible benchmark because its so dependent on underlying kernel optimizations in the Android phone market. Reply
  • tipoo - Sunday, February 24, 2013 - link

    True, other benchmarks are similarly impressive though. Reply
  • karasaj - Sunday, February 24, 2013 - link

    Psh it has nothing on my desktop! 125ms on sunspider... Nvidia so behind.

    Anyways, still looks impressive. I really want to see some Krait 600/800 benchmarks.
    Reply
  • tipoo - Sunday, February 24, 2013 - link

    The fact that they're getting well below an order of magnitude slower than desktops is impressive in itself too. Even with iPad 2 level performance I still was reluctant to do most of my web browsing on a tablet for the performance. Maybe with Tegra 4 and beyond hardware speed that will change. Reply
  • Mumrik - Sunday, February 24, 2013 - link

    As someone with heavily tabbed browsing habits, I don't think I'll ever make that jump (and I own a tablet). Reply
  • tipoo - Sunday, February 24, 2013 - link

    Also true, that's my other thing. I like to open a bunch of background tabs and have them ready as I go through each one. Right now, tablets don't do background loading, as far as I know, and if they did they wouldn't be powerful enough to keep the main tab smooth while doing it. Reply
  • Tarwin - Monday, February 25, 2013 - link

    Tablets DO do background loading, as long as they're android. The only performance I've seen is from lack of RAM on my phone and lack of bandwidth on the phone and tablet but those things affect any computer as well. One observation to ne made, they do load in the background but things like audio and video playback will pause if you switch to another tab. Reply
  • von Krupp - Monday, February 25, 2013 - link

    Even Windows Phone 7.5 and 8 do background loading. I haven't used it, but I'd wager that RT does as well, if even the gimpy mobile OS can. Reply
  • tuxRoller - Sunday, February 24, 2013 - link

    As someone who had, until recently, over 40 tabs open on my chrome browser (Nexus 4), the critical problem has been memory. With enough memory, and good enough task management, these problems tend to go away.
    Of course, maybe you are than 0.00001% who has hundreds or thousands of tabs open in which case I pity any computer you are likely to own.
    Reply

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