Core i7 vs. Core i5: Understanding the Power Story

Between generations Apple constantly struggles between squeezing every last ounce of max performance out of silicon and reducing system temperatures. I believe Apple's philosophy here is that most of the time your CPU should be running at relatively low utilization and as a result offering the full dynamic range of CPU performance is preferred to clamping max performance in order to preserve lower thermals. The problem is that in some cases, lazy background task management (e.g. keeping too many Safari windows open with Flash active) can drive CPU usage and thermals up even if you're actively doing nothing on the machine. This scenario coupled with Haswell ULT's excellent idle power consumption I believe are primary motivators for Mavericks' App Nap and occluded window slumber features.

 

 

To understand the impact on thermals (and battery life) of the Core i7-4650U on the 13-inch MacBook Air you need to understand what's going on under the hood. To hit higher frequencies, the i7-4650U generally requires a higher voltage. Power consumption (and thus thermal dissipation) can scale linearly with frequency, but it scales quadratically with voltage. The combination of the two is quite possibly the worst case scenario from a power consumption standpoint. This is why it's generally always best to increase performance via process shrinks or architectural enhancements vs. simply scaling frequency. In the case of the i7-4650U we're not talking about huge frequency/voltage scaling here, but rather a tradeoff between added performance and increased power consumption. In the table below I noted typical CPU core voltages for a couple of different operating modes on my i5-4250U and i7-4650U samples. Several years ago Intel introduced voltage binning even at a given frequency, so the voltages you see in the table below are only applicable to my parts (or other similar parts) - you could see a range of acceptable voltages in other binned parts even carrying the same model number. The values in parantheses indicate the CPU frequency (or frequencies) observed during the workload.

13-inch MacBook Air (Mid 2013) CPU Comparison - Observed Voltages
  Idle Cinebench 11.5 (1 thread) Cinebench 11.5 (4 threads)
Intel Core i5-4250U 0.665V
(800MHz)
0.852V - 0.904V
(2.3GHz - 2.6GHz*)
0.842V
(2.3GHz)
Intel Core i7-4650U 0.655V
(800MHz)
0.949V - 1.041V
(2.9GHz - 3.3GHz*)
0.786V - 0.949V
(2.8GHz - 2.9GHz*)

There are a bunch of observations here. First off, the two parts are very comparable at idle - this is how Apple can quote all implementations of the MacBook Air as being capable of up to 12 hours of battery life. At idle large parts of the silicon are clock gated if not fully power gated. Idle voltages are extremely low (even compared to what you find in modern smartphones) and both parts run at the same 800MHz frequency at idle, so power consumption is comparable between the two at idle.

Using Cinebench 11.5, I ramped up a FP intensive single threaded workload. FP workloads tend to force a bunch of large units into switching making this a great test for voltage scaling. Here we see that the i5-4250U is capable of hitting its max turbo frequency but for the most part it hangs out around 2.3GHz. The same is true for the i7-4650U, 3.3GHz is possible but most of the time it's sitting down at 2.9GHz. The i7-4650U needs higher voltages all around to hit these higher frequencies.

Next, I cranked up the number of threads. First you'll notice a reduction in clock speeds and voltages. This is where multithreading can actually be good for power consumption. Running more cores at a lower voltage for a shorter period of time can reduce total energy consumed while performing a task. The i5-4250U has no issues running at its max DC turbo frequency (2.3GHz), while the i7-4650U mostly sticks to 2.8GHz with occasional bursts up to 2.9GHz. Note that the 4650U's min voltage at 2.8GHz is actually lower than the 4250U's here. In order to hit these higher frequencies within the same TDP, Intel does have to bin for parts that do a bit better at higher frequencies whereas to make the cut for a 4250U the leakage requirements aren't as severe.

There shouldn't be any surprises thus far, but this data should give us an indication of what we can expect in terms of battery life and thermals. Where the i7 vs i5 comparison becomes tricky is if you look at workloads that can complete quick enough due to the faster performance in order to offset any additional power consumption.

CPU Performance Battery Life & Thermals
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  • abazigal - Friday, July 05, 2013 - link

    And yet for all their money, Sony still can't make a proper working trackpad. Reply
  • deasys - Friday, July 05, 2013 - link

    The Viao Pro 11 weighs less, is the SAME thickness, and has significantly LOWER battery life than the MacBook Air 11". It also lacks 802.11ac WiFi, has a flexy keyboard, poor trackpad, and shaky screen. Read the reviews.

    Finally, Boot Camp *officially* supports Windows 8.
    Reply
  • solmaker - Saturday, July 06, 2013 - link

    Moreover, the Viao Pro 11 has the old-gen Intel HD 4000 series graphics, not the spiffy new HD 5000 like the MBA. Reply
  • mikael.skytter - Friday, July 05, 2013 - link

    Hello!
    As always, thank you for the insights.

    Just a question or rather I am curious about your thoughts on the race to sleep vs battery life using the faster processor.
    I can see that under full load it draws much power. But do you have any thoughts regarding the "normal" usage scenario where the processor is powering up, performing a task and then goes back to sleep on the faster/slower processor?

    Best Regards,
    Mikael
    Reply
  • abazigal - Friday, July 05, 2013 - link

    It would seem that Apple is further trying to differentiate between its air and pro lines. The macbook air's processor likely suffices for most everyday tasks, and you can't go wrong with longer battery life. They are also quite affordable, for those shopping on a budget.

    The retina pros come with faster processors and a vastly superior screen for those with deeper pockets and are willing to spend on quality. I won't be surprised if the 13" pro comes later this year with up to 16gb ram possible, to distinguish it from its 13" air cousin.
    Reply
  • ananduser - Friday, July 05, 2013 - link

    Those shopping on a budget will not drop 1000$ or more for an underpowered coffee shop computer. Anything else other than browsing and video(tasks for which the latest Intel chip is highly optimized) kills the battery life and makes the entire kit reach 90 degrees. Reply
  • deasys - Friday, July 05, 2013 - link

    "Ananduser," you really should read the article before commenting on it…

    In this particular case, see the tests on Lightroom and Photoshop uses as well as the heavy workload tests. Not many would be doing that sort of stuff in a "coffee shop" and under that heavy workload, I don't think any reasonable person would characterize over 5 and half hours of battery run time as "killed."

    And by the way, "the entire kit" does not reach 90 degrees, only the CPU itself.
    Reply
  • ananduser - Saturday, July 06, 2013 - link

    I read the comment but apparently you didn't read mine. Budget shoppers(to which to OP made a reference), you know people that don't have 1000$ to spend on a cramped 11" ULV chipset, definitely have better laptop options to choose from, with just as much battery(thanks to Haswell). Those options aren't as portable but certainly don't need adapters to expand the port selection or expensive software licenses(Windows8 or VMWare). Reply
  • FwFred - Friday, July 05, 2013 - link

    The i7 MBA is pretty close in many benchmarks to the 35W 13" rMBP. It's only "underpowered" compared to quad core i7's. Can you name a tablet/PC/whatever where you wouldn't kill the battery if you keep the CPU pegged?

    It seems to me you have a very capable PC with the portability and battery life of an iPad.
    Reply
  • ananduser - Saturday, July 06, 2013 - link

    Budget shoppers, to which the OP made a reference, have better machines to spend 1000$ on. Not as portable but plenty portable still. Reply

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