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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  • GotThumbs - Monday, July 08, 2013 - link

    @dsumanik: I feel your frustration and can see what your point is about. I've actually been coming to AnandTech since Anand was in HighSchool. I also have been disappointed in what I perceive as "Marketing spin" type reviews.
    Other than swapping in an updated CPU's and SSD's, what's the REAL effort/innovation for the 2013 line? Not much, but yet how many articles have been written and what is the overall feeling of each of those "reviews"?
    I've gotten to the point where I actually find it funny/amusing that so much "pomp and circumstance" has been put into recent Apple articles. When people ask me why I don't have an Iproduct, they are totally amazed when I tell them I'd never want an IProduct. I understand it goes against the common thinkers, but I'm not a follower and I choose to avoid systems where I'm not in contol. It's my choice and I respect others decision to chose what they want.

    As far at Anand's recent "Reviews", (no disrespect to Anand as an individual), but It's like a car salesman spinning the minor differences between the new years models and last years. "Less powerful engine, but hey you don't really need/want that"

    At the end of the day, everyone has their own choice to make and while I'll never own an Apple product, its due to the overpriced and closed ecosystem Apple's created. When you're (Apple) pushing a cult like community...they (Fanbois) willingly follow blindly IMO.

    You can try to explain your point forever.....but you must realize they will never even consider your point. It' would be akin to treason for them to even acknowledge your point of view.

    Best wishes to all and enjoy your choices,
    Reply
  • amdwilliam1985 - Monday, July 08, 2013 - link

    "...I choose to avoid systems where I'm not in contol."

    Kind of wish I've read your comment 2 years earlier. I've got the 2011 13" MBA, although the overall machine feels good, somehow I can never shake the feeling that it's Steve J's labtop and I'm not "in control" of it. I use my MBA as a glorifying web browsing machine and for everything else, I go to my windows 7 machine :)
    For the same reason(iTunes), I've dumped my beloved 3gs for Galaxy S2 and S3.
    I'm probably one of the few control freaks who needs to know that I'm in control of my pc/phone, instead of someone else.
    Reply
  • TheinsanegamerN - Thursday, July 04, 2013 - link

    two things. A. 1366x768 if just fine for a 11 inch screen. 1440x900 is great for a 13 inch screen, and i wish more oems would use 16:10 screens.
    B. why would apple change a design that works? getting 12 hours of battery life, or close too it, is downright amazing for a machine this size. although, id love to see what would happen if they took current tech and put it in an older design mac, the battery life would be epic. soo much battery.
    Reply
  • Spoony - Tuesday, July 09, 2013 - link

    No, those resolutions are not fine. Of course people want 2880x1800 on their 13" notebook. Of course they want 2560x1600 on their 11" notebook. However, the MBA is an ultra-mobile platform meaning battery life and size take priority over other traits. High DPI displays have a considerable power cost, and apparently Apple felt that trading off display resolution for battery life was the right call in this situation. At some point you just can't beat physics and you have to compromise.

    If you don't agree, don't buy their product. I'm not sure I agree, so I bought something else.
    Reply
  • kleos44 - Friday, July 12, 2013 - link

    I am tired of hearing this argument. Throwing out statements like "you just can't beat physics" is wrong. Educate yourself on the physics of IGZO displays. Significantly higher resolutions AND LESS power draw than traditional display technologies. Every high end laptop is going to use them this year including pretty much every other Apple product. Reply
  • TheeGooch - Thursday, November 28, 2013 - link

    Well said. I've slowly went from anti-Apple to being a fan over the last 7 years. 2014 will be when I buy my first Apple computer because the stability, quality, and ecosystem that comes with Apple products. The design choices they make to balance price, functionality, and quality just jive with my own idea of how those should be balanced. It's weird to think back to when I hated all things Apple on principle. Once I got my 1st Apple product( the ipod) , that began to change. Reply
  • ESC2000 - Monday, August 12, 2013 - link

    It is not fine when it is on a $1,000+ computer. That is a premium price, and the display, with its low resolution and TN panel, belongs on a low end machine. Apple always does this: either the device is way overpriced or some feature is skimped on.

    There is no reason that this machine, with its poor display and processing sometimes shower than last year's version, should cost as much or more than a similar windows machine even though apple does not have to pay for a software license. All that money is just going to apple's bottom line. I never understand when prior brag about how apple is the most profitable: all that means is you are being overcharged.
    Reply
  • zh.aung - Friday, July 05, 2013 - link

    If you hate apples, go and eat oranges. Don't waste your time writing this junk. Reply
  • KoolAidMan1 - Friday, July 05, 2013 - link

    Haha, how much angry spittle is on your monitor right now?

    Funniest post I've read in a LONG time, thanks for the laughs
    Reply
  • robco - Friday, July 05, 2013 - link

    If you're going to say pay back's a bitch, note the apostrophe. It is a contraction for payback is a bitch. Nobody will take you seriously if you can't even write a coherent sentence.

    As noted in the review, performance is on par with the previous generation, but with substantially better battery life. For many users, this is a huge bonus. For those wanting more performance, the i7 upgrade is reasonable, and will not adversely impact battery life to a great degree.

    Apple's design has been copied, unsuccessfully, by several other manufacturers. The MBA remains one of the few ultrabooks to have a decent display, keyboard, and touchpad.

    Plenty of other ultrabooks lack a celluar modem. There isn't a lot of free space.

    The CEO of Apple is Tim Cook. Do a little research.

    The next version of OS X, which purchasers of the MBA will receive gratis, may in fact fix some of the issues. Of course, you could look at the review of the Razer Blade, a new gaming Windows laptop that comes with neither 802.11ac or the ability to add gig-E, something the MBA can do by virtue of its (correctly leaving out the apostrophe here as it's possessive and not a contraction) Thunderbolt port.

    So yes, the fact remains that the MBA has great battery life, fast networking, a good (admittedly not great) display, solid keyboard, and great touchpad. OS X continues to provide far superior battery life than Windows. If you must pollute the internet with this sort of screed, learn to write properly and do you research.
    Reply

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