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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  • youwonder - Wednesday, October 17, 2012 - link

    I find it kind of ...odd that the S3 has a much larger battery than the one X and the same SoC yet posts significantly worse LTE browsing numbers, and is the only phone using LTE to get worse results with it than using 3G(granted that is the international vers, doesn't look like they had time to do testing on the AT&T or verizon variant running 3G). Does the samoled screen really draw THAT much more power than an LCD? also there's this which makes me wonder more:

    http://blogs.which.co.uk/technology/smartphones/be...

    Of course, I don't respect these guys as much as anandtech when it comes to accurate results, and they did things much differently (broadcasting their own 3g signal and putting all phones on max brightness), but still the odd results here make me wonder if a small mistake wasn't made.
    Reply
  • Zink - Wednesday, October 17, 2012 - link

    Max brightness gives the gs3 an advantage because its screen is so dim. The other phones are using LED lighting as well but they go much brighter and have to shine through the LCD panel. Reply
  • youwonder - Wednesday, October 17, 2012 - link

    Good point, I guess it's mostly just me wondering why the GS3 LTE variant posts such horrible numbers even compared to it's 3G version when anand specs a good amount of time explaining why the opposite is true. Reply
  • phillyry - Sunday, October 21, 2012 - link

    Don't know why but it does tank on LTE. Reply
  • rarson - Wednesday, October 17, 2012 - link

    I'm getting so sick and tired of seeing the word "literally" injected into all sorts of sentences that it doesn't belong in. This word only needs to be used when describing something literal. It's not a synonym for "really" (not yet, anyway). Reply
  • andykins - Wednesday, October 17, 2012 - link

    Alright, language purist. :P Reply
  • joos2000 - Thursday, October 18, 2012 - link

    http://theoatmeal.com/comics/literally Reply
  • phillyry - Sunday, October 21, 2012 - link

    Great link. That's too funny - literally! Reply
  • dfonseca - Wednesday, October 17, 2012 - link

    On the last page, section "Final Words" / "iPhone 5 Device Conclusions", it's written:

    > At a high level, the iPhone 5’s cameras appeared to be some of the least unchanged elements of the new device however in practice the improvements are significant.

    "Least unchanged" means "most changed." It should probably say "most unchanged," or "least changed."

    Nice review, kudos to all authors.
    Reply
  • mattlach - Wednesday, October 17, 2012 - link

    I had the original iPhone, followed by the iPhone 3G and then the iPhone 4, and just switched to a Samsung Galaxy S3 in July.

    When the original iPhone came out, while it was the first to do what it did - and that's why I bought it at its steep no-contract introductory price - it wasn't exactly revolutionary, everything in the market was moving in this direction, but it was pretty well executed and nothing else did it at the time.

    I upgraded to the 3G on launch, as I thought the edge speeds were dreadful, but was disappointed, as the phone wasn't fast enough to take advantage of 3G, and AT&T's 3G was pretty mediocre anyway. It didn't get important features its competitors had, like copy and paste until very late in the game, and I started to think that I should have gotten an Android phone instead.

    By the time I got the iPhone 4, I was tired of my slow 3G experience and just wanted an upgrade to something faster. The iPhone 4 was a good upgrade, but I really only got it because I didn't like AT&T's Android offerings at the time. I had been thinking about going to Verizon and getting an Android for some time. The 3G should have been my last iPhone, it was a mistake to buy the 4.

    Having realized my mistake, I waited 2 long years with the 4 until I could finally get out of my AT&T contract and go to Verizon and get a GS3, and it felt great.

    The additional freedom of what I run on my phone, not being controlled by Apple and their agenda as to what makes it into the App store, and the fact that I finally no longer had to have iTunes installed on my computer were fantastic.

    My computer has been iTunes free for 3 months now, and it feels great!

    I was concerned for a while that once the iPhone 5 was released, they would come out with something that would make me regret my choice of the GS3, but it turns out they didn't.

    I'll likely never buy anything Apple again. It feels like a huge relief to say that.
    Reply

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