Absolute Power


iPad 2 too

 This blog originally posted on the Low Power Engineering Community 3/17/10. 

Our stable of test devices continues to expand.  After only a couple of hours in line at the Stanford Shopping Center last Friday, I have my iPad 2, complete with a funky lime-green cover! It’s the low-end 16 GB model, with wifi connectivity only. After our recent experiments on power efficiency versus communications, I figure I’d rather isolate the biggest energy leak in the system by using my iPhone 4 as the communications hub, and go for the new Personal Hotspot feature (replacing MyWi) to get mobile data to other devices. Draining batteries on multiple devices in parallel just doesn’t make any sense to me.

So our test device is a white 16GB iPad 2, complete with two cameras, gyroscope, and a new processor, and is thinner and lighter than the original.  Not bad right out of the gate.  I also got the two add-ons: a magical magnetic cover, and the new HDMI cable.

First, the cover – I thought when I bought it that $39 was a ridiculous price to pay for a one-side cover, but after trying it out, I love it!  It has a whole slew of magnets – 21 in the cover, and 10 in the iPad 2 itself (check out the “iPad 2 cover teardown” on iFixit) that all work together to allow the cover to work as a typing stand, a display stand, a screen cleaner, and (of all things) a cover.  Totally gadgety – right up my alley.  The other add-on is the HDMI cable – this is a HUGE improvement over all of the original cables and interfaces (VGA, component, HDMI, DVI, composite) that Apple and third parties offered.  Not because the cable itself is better or worse, but because the iPad 2 now supports full-time mirroring through HDMI, straight onto your big screen TV! Netflix looks great, presentations are useful, surfing the web is fun, and a little (err, BIG) “Pig Rush” certainly passes the time nicely. Thumbs up on the add-ons!

I haven’t looked much at power efficiency of the cameras or gyroscope subsystems, so let’s jump straight to the processor. The original iPad used a 1 GHz A4 processor, based on the ARM Coretex A8.  The iPad 2 utilizes the new A5 processor, built upon the ARM Coretex A9, with dual processor cores at 1 GHz, and clock speed variability to save power. There are many teardowns and reviews of the A5 processor for more gory details – one of my favorites is at AnandTech (a great site if you’re interested in hardware design and analysis by the way). Bottom line from all of the benchmarks that have been run so far is that the A5 in the iPad 2 can nearly double the performance in real apps, and possibly up to 6-8x performance for floating point intensive apps compared to the A4.  That’s quite a boost in horsepower, and the most interesting question to me is, “how much does it cost me in power efficiency?”

There is clearly plenty of performance headroom in the iPad 2.  But what does it cost in terms of energy?  A 400 hp car might accelerate twice as quickly as a 200 hp car, but in many cases it also gets about half the mpg – carrying around the extra weight of a more powerful engine is typically a drain on efficiency.  In our case, my original iPad power efficiency tests showed about 3.2 Wh of energy required to view the Star Trek movie (2:06:46) in the “maximum battery life” configuration (airplane mode, brightness & sound minimum).  Since updating the software to iOS 4.2.1, that number has dropped to 2.73 Wh, a 15% improvement! There were some rumored improvements in power performance in iOS 4.2 – I’m still checking into the details on this.  For the iPad 2 in the same configuration running iOS 4.3, the energy required is just 2.25 Wh.  Power efficiency on the iPad 2 has increased enough to more than make up for the “bigger engine”!  That’s pretty incredible.  One clue is that the A5 processor is manufactured using a 40 nm process, compared to the 45 nm process of the A4.

Similarly, in maximum movie watching mode (display and sound at maximum), the iOS 4.1 iPad required 6.2 Wh, updated to iOS 4.2 it requires 5.95 Wh, and the new iPad 2 requires just 5.75 Wh for the Star Trek movie test.  Nice improvements in power efficiency all around.

Many people have asked me if the iPad 2 outperforms the original iPad.  The answer, of course, is “not yet”.  That’s because virtually every iPad app out there is designed to run on the original iPad, and in general, software developers (the good ones anyway), have tuned performance, and even limited functionality, to suit their target platform.  Six months from now, when the latest apps require the additional performance of the iPad 2, the original will undoubtedly feel a little sluggish.

In fact, I have no complaints about performance on my iPad, so I wouldn’t expect new hardware to suddenly seem much faster. This behavior is a testament that device performance today isn’t the bottleneck – there’s plenty of horsepower to accomplish amazing things in today’s apps. What will change down the road is that apps will get even more amazing.  I’m still waiting for the “headless man” Halloween costume app, which would utilize two iPads, one in front and one behind your head, connected via FaceTime, effectively making your head “invisible”! You have to think about it a bit, but this might be the first cut at the famed Romulan Cloaking Device! 


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