Increased Dynamic Range: Understanding the Power Profile of Modern SoCs

Section by Anand Shimpi

The iPhone 4S greatly complicated the matter of smartphone power consumption. With the A5 SoC Apple introduced a much wider dynamic range of power consumption to the iPhone than we were previously used to. Depending on the workload, the A5 SoC could either use much more power than its predecessor or enjoy decreased overall energy usage. I began our battery life analysis last time with some graphs showing the power savings realized by a more power hungry, faster CPU.

The iPhone 5 doesn't simplify things any more. I believe the days of us having straightforward discussions about better/worse battery life are long gone. We are now firmly in the era of expanded dynamic range when it comes to smartphone power consumption. What do I mean by that? The best way to explain is to look at some data. The graphs below show total device power consumption over time for a handful of devices running the Mozilla Kraken javascript benchmark. Kraken is multithreaded and hits the CPU cores fairly well. The power profile of the benchmark ends up being very similar to loading a very js-heavy web page, although for a longer period of time. All of the device displays were calibrated to 200 nits, although obviously larger displays can consume more power.

Let's start out by just looking at the three most recent iPhone generations:

The timescale for this chart is just how long the iPhone 4 takes to complete the Kraken benchmark. The iPhone 4/4S performance gap feels a lot bigger now going back to the 4 than it did when the 4S launched, but that's how it usually seems to work. Note how tight the swings are between min and max power consumption on the iPhone 4 during the test. As a standalone device you might view the iPhone 4 as being fairly variable when it comes to power consumption but compared to the 4S and 5 it might as well be a straight line.

The 4S complicated things by consuming tangibly more power under load than the 4, but being fast enough to complete tasks in appreciably less time. In the case of this Kraken run, the 4S consumes more power than the 4, however it's able to go to sleep quicker than the 4 and thus draw less power. If we extended the timeline for the iPhone 4 significantly beyond the end of its benchmark run we'd see the 4S eventually come out ahead in battery life as it was able to race to sleep quicker. The reality is that with more performance comes increased device usage - in other words, it's highly unlikely that with a 50% gain in performance users are simply going to continue to use their smartphone the same way as they would a slower device. Usage (and thus workload) doesn't remain constant, it's somewhat related to response time.

The iPhone 5 brings new meaning to device level power consumption. With a larger display and much more powerful CPU, it can easily draw 33% more power than the 4S under load, on average. Note the big swings in power consumption during the test. The A6 SoC appears to be more aggressive in transitioning down to idle states than any previous Apple SoC, which makes sense given how much higher its peak power consumption can be. Looking at total energy consumed however, the iPhone 5 clearly has the ability to be more power efficient on battery. The 5 drops down to iPhone 4 levels of idle power consumption in roughly half the time of the iPhone 4S. Given the same workload that doesn't run indefinitely (or nearly indefinitely), the iPhone 5 will outlast the iPhone 4S on a single charge. Keep the device pegged however and it will die quicker.

Out of curiosity I wanted to toss in a couple of other devices based on NVIDIA and Qualcomm silicon to see how things change. I grabbed both versions of the HTC One X:

The Tegra 3 based One X actually performs very well in this test, but its peak power consumption is significantly worse than everything else. It makes sense given the many ARM Cortex A9 cores built on a 40nm G process running at high clock speeds on the Tegra 3.

The 28nm Snapdragon S4 (dual-core Krait) based One X gives us some very interesting results. Peak power consumption looks identical to the iPhone 5, however Apple is able to go into deeper sleep states than HTC can with its S4 platform. Performance is a little worse here but that could be a combination of SoC and software/browser. I used Chrome for all of the tests so it should be putting Android's best foot forward, but the latest update to Safari in iOS 6 really did boost javascript performance to almost untouchable levels.

At the end of the day, the power profile of the iPhone 5 appears to be very close to that of a modern Snapdragon S4 based Android smartphone. Any battery life gains that Apple sees are strictly as a result of software optimizations that lead to better performance or the ability to push aggressively to lower idle power states (or both). It shouldn't be very surprising that these sound like a lot of the same advantages Apple has when talking about Mac battery life as well. Don't let the CPU cores go to sleep and Apple behaves similarly to other device vendors, but it's really in idle time or periods of lighter usage that Apple is able to make up a lot of ground.

There's one member of the modern mobile SoC market that we haven't looked at thus far: Intel's Medfield. The data below isn't directly comparable to the data above, my measurement methods were a little different but the idea is similar - we're looking at device level power consumption over time while Kraken runs. Here I'm only focusing on the latest and greatest, the Atom based Motorola RAZR i, the Snapdragon S4 based Droid RAZR M and the iPhone 5. The RAZR i/M are nearly identical devices making this the perfect power profile comparison of Atom vs. Snapdragon S4. The RAZR i is also the first Atom Z2460 based part to turbo up to 2.0GHz.

Very interesting. Atom is the only CPU that can complete the Kraken benchmark in less time than Apple's Swift. Peak power consumption is definitely higher than both the Qualcomm and Apple devices, although Intel's philosophy is likely that the added power usage is worth it given the quicker transition to idle. Note that Atom is able to drive to a slightly lower idle level than the Snapdragon S4, although the Swift based iPhone 5 can still go lower.

At least based on this data, it looks like Intel is the closest to offering a real competitor to Apple's own platform from a power efficiency standpoint. We're a couple quarters away from seeing the next generation of mobile SoCs so anything can happen next round, but I can't stress enough that the x86 power myth has been busted at this point.

I will add that despite Intel's performance advantage here, I'm not sure it justifies the additional peak power consumption. The RAZR i ends up being faster than the iPhone 5 but it draws substantially more power in doing so, and the time savings may not necessarily offset that. We'll see what happens when we get to our battery life tests.

 

GPU Analysis/Performance Battery Life
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  • TrackSmart - Wednesday, October 17, 2012 - link

    Shouldn't the battery life on the Verizon Galaxy SIII with LTE be higher than shown? That result bunches up with the 3G scores rather than the LTE scores. I wonder if the "LTE" listing is a typo.

    It's also a bit surprising how much of a difference the connection speed makes (3G vs 4G LTE) for the battery life tests. Are you guys really testing differences in power efficiency under typical use? Or have you, inadvertently, created a strange test of air interface throughput/watt - which would vary based on signal strength and network speed, but not based on the main device power draw under typical browsing (i.e. screen + intermittent CPU usage spikes).

    I would have guessed that that screen power draw would be the largest cause for differences between handsets, not the air interfaces on the new devices, now that LTE is no longer a power hog.
    Reply
  • phillyry - Sunday, October 21, 2012 - link

    If it's anything like the Telus GS3 up here in Canada than it's not likely a typo. My brother has it and it tanks so bad on LTE that he keeps it turned off. Same thing with NFC btw. Two major selling features of the GS3 that went down the tubes in reality. Reply
  • Skidmarks - Wednesday, October 17, 2012 - link

    I've got to hand it to Apple, if nothing else they sure know to market their rubbish. Reply
  • Freakie - Wednesday, October 17, 2012 - link

    I haven't seen the 5 in person, but every time I see a picture of the front of it, I swear its design echos Samsung devices quite a bit. If any light hits it directly then it looks off-black, at least in the pictures, to the point of looking like Samsung's Pebble Blue color back when it was a bit darker (Samsung Impression).

    They got rid of the band around the phone and just have a slanted surface which when looking at pictures taken 8 inches away from the phone, has it's sharp "edges" that it slants to become one smooth transition. Reminds me a lot of the GSII's front.

    Now I'm not a fan of any design litigations going either way, but I've never seen a Samsung device echo the looks of Apple's devices quite as much as the iPhone 5 echos a number of Samsung's design flairs that they've been using for a while.

    Just my two cents xP
    Reply
  • kmmatney - Wednesday, October 17, 2012 - link

    Are you kidding me. Look at how much the original; Samsung Galaxy copies the iPhone 3G. Same with the Galaxy SII.

    See for yourself

    3G versus SGI

    https://encrypted-tbn1.gstatic.com/images?q=tbn:AN...

    and 4S versus SGII

    http://www.gizmowatch.com/entry/comparing-mights-i...
    Reply
  • medi01 - Wednesday, October 17, 2012 - link

    Wow, SGII and 4S have the same screen ratio, you shameless iScum... Reply
  • Freakie - Wednesday, October 17, 2012 - link

    Oh I never said that Samsung hasn't produced a phone similar to an iPhone, though your second picture is pretty ridiculous (that's the SGI not SGII) which came out several months before the 4. Not only that but the picture its self is most definitely shot/edited in a way to make them as similar as possible.

    My complaint is just Apple doing something very different than normal, and echoing someone else for a change. Usually they seem to go for something different than the rest and the iPhone 5 most definitely does not come anywhere near that.
    Reply
  • steven75 - Wednesday, October 17, 2012 - link

    This is probably the most silly comment I've read among any iPhone reviews. iPhones have always been similar overall since the revolution if 2007 and now you are seriously making the claim this looks more like a Samsung device?

    /smh
    Reply
  • MNSoils - Wednesday, October 17, 2012 - link

    Apple has an interesting story here and your group did a wonderful job telling it.

    On the 2 graph of the "Increased Dynamic Range" page, the idle power for the Tegra 3 SOC after finishing the Kraken benchmark seems awfully high for just the companion core. Does more time have to elapse before Android reverts to the companion core? Is the companion core not that power efficient (power-gated, etc.)? Does Android revert to the companion core?
    Reply
  • colonelclaw - Wednesday, October 17, 2012 - link

    Thanks for a terrific article. It's just a shame that about 75% of the comments will be by people who either love the device or hate it, and nothing in this carefully researched and written appraisal will make them change their minds either way.

    How did we get to this? Actually, don't answer, that would turn into an irrelevant pissing match too.
    Reply

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