Modifying a Krait Platform: More Complicated

Modifying the Dell XPS 10 is a little more difficult than Acer's W510 and Surface RT. In both of those products there was only a single inductor in the path from the battery to the CPU block of the SoC. The XPS 10 uses a dual-core Qualcomm solution however. Ever since Qualcomm started doing multi-core designs it has opted to use independent frequency and voltage planes for each core. While all of the A9s in Tegra 3 and both of the Atom cores used in the Z2760 run at the same frequency/voltage, each Krait core in the APQ8060A can run at its own voltage and frequency. As a result, there are two power delivery circuits that are needed to feed the CPU cores. I've highlighted the two inductors Intel lifted in orange:

Each inductor was lifted and wired with a 20 mΩ resistor in series. The voltage drop across the 20 mΩ resistor was measured and used to calculate CPU core power consumption in real time. Unless otherwise stated, the graphs here represent the total power drawn by both CPU cores.

Unfortunately, that's not all that's necessary to accurately measure Qualcomm CPU power. If you remember back to our original Krait architecture article you'll know that Qualcomm puts its L2 cache on a separate voltage and frequency plane. While the CPU cores in this case can run at up to 1.5GHz, the L2 cache tops out at 1.3GHz. I remembered this little fact late in the testing process, and we haven't yet found the power delivery circuit responsible for Krait's L2 cache. As a result, the CPU specific numbers for Qualcomm exclude any power consumed by the L2 cache. The total platform power numbers do include it however as they are measured at the battery.

The larger inductor in yellow feeds the GPU and it's instrumented using another 20 mΩ resistor.

Visualizing Krait's Multiple Power/Frequency Domains

Qualcomm remains adament about its asynchronous clocking with multiple voltage planes. The graph below shows power draw broken down by each core while running SunSpider:

SunSpider is a great benchmark to showcase exactly why Qualcomm has each core running on its own power/frequency plane. For a mixed workload like this, the second core isn't totally idle/power gated but it isn't exactly super active either. If both cores were tied to the same voltage/frequency, the second core would have higher leakage current than in this case. The counter argument would be that if you ran the second core at its max frequency as well it would be able to complete its task quicker and go to sleep, drawing little to no power. The second approach would require a very fast microcontroller to switch between v/f modes and it's unclear which of the two would offer better power savings. It's just nice to be able to visualize exactly why Qualcomm does what it does here.

On the other end of the spectrum however is a benchmark like Kraken, where both cores are fairly active and the workload is balanced across both cores:

 

Here there's no real benefit to having two independent voltage/frequency planes, both cores would be served fine by running at the same voltage and frequency. Qualcomm would argue that the Kraken case is rare (single threaded performance still dominates most user experience), and the power savings in situations like SunSpider are what make asynchronous clocking worth it. This is a much bigger philosophical debate that would require far more than a couple of graphs to support and it's not one that I want to get into here. I suspect that given its current power management architecture, Qualcomm likely picked the best solution possible for delivering the best possible power consumption. It's more effort to manage multiple power/frequency domains, effort that I doubt Qualcomm would put in without seeing some benefit over the alternative. That being said, what works best for a Qualcomm SoC isn't necessarily what's best for a different architecture.

Introduction Krait: Idle Power
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  • djgandy - Friday, January 04, 2013 - link

    People care about battery life though. If you can run faster and go idle lower you can save more power.

    The next few years will be interesting and once everyone is on the same process, there will be less variables to find to assert who has the most efficient SOC.
    Reply
  • DesktopMan - Friday, January 04, 2013 - link

    "and once everyone is on the same process"

    Intel will keep their fabs so unless everybody else suddenly start using theirs it doesn't look like this will ever happen. Even at the same transistor size there are large differences between fab methods.
    Reply
  • jemima puddle-duck - Friday, January 04, 2013 - link

    Everyone cares about battery life, but it would take orders of magnitudes of improvement for people to actually go out of their way and demand it. Reply
  • Wolfpup - Friday, January 04, 2013 - link

    No it wouldn't. People want new devices all the time with far less.

    And Atom swaps in for ARM pretty easily on Android, and is actually a huge selling point on the Windows side, given it can just plain do a lot more than ARM.
    Reply
  • DesktopMan - Friday, January 04, 2013 - link

    The same power tests during hardware based video playback would also be very useful. I'm disappointed in the playback time I get on the Nexus 10, and I'm not sure if I should blame the display, the SOC, or both. Reply
  • djgandy - Friday, January 04, 2013 - link

    It's probably the display. Video decode usually shuts most things off except the video decoder. Anand has already done Video decode analysis in other articles. Reply
  • jwcalla - Friday, January 04, 2013 - link

    You can check your battery usage meter to verify, but... in typical usage, the display takes up by far the largest swath of power. And in standby, it's the wi-fi and cell radios hitting the battery the most.

    So SoC power efficiency is important, but the SoC is rarely the top offender.
    Reply
  • Drazick - Friday, January 04, 2013 - link

    Why don't you keep it updated? Reply
  • iwod - Friday, January 04, 2013 - link

    I dont think no one, or no anandtech reader with some technical knowledge in its mind, has ever doubt what Intel is able to come up with. A low power, similar performance or even better SoC in both aspect. Give it time Intel will get there. I dont think anyone should disagree with that.

    But i dont think that is Intel's problem at all. It is how they are going to sell this chip when Apple and Samsung are making one themselves for less then $20. Samsung owns nearly majority of the Android Market, Which means there is zero chance they are using a Intel SoC since they design AND manufacture the chip all by themselves. And when Samsung owns the top end of the market, the lower end are being filled by EVEN cheaper ARM SoCs.

    So while Intel may have the best SoC 5 years down the road, I just dont see how they fit in in Smartphone Market. ( Tablet would be a different story and they should do alright.... )
    Reply
  • jemima puddle-duck - Friday, January 04, 2013 - link

    Exactly. Sometimes, whilst I enjoy reading these articles, it feels like the "How many angels can dance on the head of a pin" argument. Everyone knows Intel will come up with the fastest processor eventually. But why are we always told to wait for the next generation? It's just PR. Enjoyable PR, but PR none the less. Reply

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