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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  • lilo777 - Saturday, July 06, 2013 - link

    MBA will soon be the only laptop in its class that still uses TN panel. And for a "best computer" it has a lauphably low resolution. Lack of 4G is annoying. Lack of touch screen option is inexcusable. In short, MBA is a solid mid-range laptop. More importantly OP made a very valid observation that lately AT stated giving Apple products way more credit than they deserve. Reply
  • vFunct - Sunday, July 07, 2013 - link

    No the MBA is perfect. Nobody wants a retina screen at that size, since the only people that use them are photographers and they have MBPros. No one wants 4g either since they tether anyways. And touch-screens are completely useless, so no one wants them either.

    Again, Apple makes the best laptops in the industry. They are perfect for their target audiences.
    Reply
  • lilo777 - Sunday, July 07, 2013 - link

    So the photographers want "retina" displays on the phones , iPads and MBPs but not MBAs. interesting. Also, what kind of photographers prefer TN panels to IPS panels? Reply
  • vFunct - Monday, July 08, 2013 - link

    Photographers use iPhones, iPads, and MacBook Pros. They DON'T use MacBook Airs.

    The people that buy MacBook Airs use them for email, office apps, and surfing the web. Those users have never wanted or cared about retina displays.

    So, this Retina display complaint is a non-complaint, since no one that would want a MacBook Air is going to complain about it.

    You guys need to learn the purpose of what you use a computer for.

    You don't buy a computer to own tech specs. You buy a computer as a tool to do what you want, and the people that want the MacBook Airs do not want a Retina display.
    Reply
  • icoltsfan94 - Tuesday, August 13, 2013 - link

    I disagree in the fact that MacBook Air potential buyers would not want a retina display. I personally have considered a MBA and one of the biggest turn-offs has been its 1366x768 display, even an upgrade to a 1920x1080 display would be an amazing upgrade, after all, the prevalence of HD content on the internet has significantly increased since the incarnation of the MBA, which has that same 1366x768 resolution at its release. Not to mention the ability to have multiple web pages open at the same time on one screen. Even then, if you have no desire for any of these items, having a higher resolution in the same screen size would allow for better rendered fonts and images in web pages even in just general web browsing. I would love to see an honest argument against having a higher resolution display in the MBA. Reply
  • bji - Sunday, July 07, 2013 - link

    Is "no touch screen option" actually a valid complaint about any laptop? Reply
  • gentux - Friday, July 26, 2013 - link

    Well the best display is useless if it has no juice left. Even Sony Vaio Pro has not that much battery power in it yet it's more expensive. Speaking of 4G you know how many different bands and providers there are in the world? And how that changes in the next 3 years a typical notebook is supposed to be used? It's better to keep that separate so you can use some tethering solutions abroad. Reply
  • GotThumbs - Monday, July 08, 2013 - link

    It's not hard when you (Apple) controls every aspect of its ecosystem. Software, OS, Hardware and marketplace are owned and controlled by Apple. You cannot load Apples OS onto any other hardware (Legally anyway), so they control/limit the hardware configuration and thus the number of drivers you need to have a stable system. Google Chrome Books actually are a similar option (IMO) and with more companies moving to cloud based software.....there is less need to actually install programs on your desktop/laptop.

    If MS decided to regulate Windows on systems of thier own design, you would have very stable systems from MS as well. Just simply think of the number of combinations of PC systems /components currently available. It's in the millions. How many models does Apple have? What are the differences between each one?

    Jobs was very smart, but I have as much respect for him as I do a sleazy car salesmen IMO. (Google: Steve Jobs, Wozniak, Breakout) . While I will never own an IProduct, I give the company credit for how much money they have raked in while offering very limited product lines and controlling the content market where the REAL money is to be made. Apple has made billions selling one phone with incremental improvements to its fan base. What other company wouldn't want to follow that kind of success? Thats why I believe MS has initiated mandatory accounts to load apps onto Win8. All businesses exist to make money for the investors/owners. Apple is NOT any different IMO.

    Best wishes,
    Reply
  • gentux - Friday, July 26, 2013 - link

    Actually Microsoft has a logo program that doesn't help make PCs any more stable. Just as PlaysForSure stood for something that's a joke since the failed Zune. Sorry but Microsoft wasn't good at anything. Nobody really cared about Windows until they forced you to buy it. Then everyone hated it until Apple had their systems and except for bigger companies and people who think they need to be other than everyone else are moving to OS X.

    If you ask people who used both they will stay with OS X that's the success Apple did.

    If that's not a big of a point why there's no one else?
    Reply
  • ESC2000 - Monday, August 12, 2013 - link

    If that was true - that people who have used both windows and OSX always stick with apple - apple's marketshare would be way higher. In the computer space its marketshare is puny even though it offers lower cost laptops than it used to. I count myself as one of the people who stuck with Windows. I had to use my sister's macbook pro for a semester in college after I spilled on my VAIO. That was a miserable semester for me computer-wise... couldn't wait to get back to windows. I am far from the only person who prefers windows. I see it when i look around the classroom and see way more windows computers (mostly high-end so people who could have bought the air at least) and I see it in every company/firm that I have ever worked for or even visited -literally none used OSX. So good try there.

    I get this shtick all the time on these tech sites: 'anybody who has ever used [insert iproduct here] will prefer it over other options and never go back to anything else.' We all know logically that absolutist statements like that cannot be true (saw another one a page back claiming that apple makes the best computers period). All one need do to disprove it is if course provide one example to the contrary.

    I know most of these statements are meant somewhat hyperbolically (although I also think people are apt to project their own experience onto every other person in the world). to the extent that they merely mean the trend is toward people abandoning windows for OSX, all I can say is it is not what I observe - whether computers, tablets (v. Android), or phones (again v. Android). But of course we see what we want to see and what we are looking for, so without hard numbers it is hard to know.
    Reply

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