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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  • FwFred - Saturday, July 06, 2013 - link

    OP also called the MBA a coffee shop computer. While it certainly excels at this, it is in no way limited to this. Reply
  • KitsuneKnight - Friday, July 05, 2013 - link

    Underpowered? I don't have the 2013 Air, but the base 13" 2011 model, and it's certainly not underpowered by any stretch of the imagination.

    It makes a wonderful development machine. I regularly run Xcode (+ iOS Simulator), Netbeans, and Visual Studio Premium 2012 (in a Windows 8 VM)*. None of which have a significant impact on battery life (even Visual Studio + Windows 8, which I definitely didn't expect), either. If you want a very light computer for software development, the Air is a wonderful machine.

    The only things that actually "kills" battery life are OpenGL games (usually knocking it down to 2-3 hours, from 7). The machine DOES have a good enough GPU to play some indie titles like Minecraft and Kerbel Space Program comfortably, but I'd never recommend an Air as a gaming machine.

    * In addition to any of those, I also always have open Safari, Chrome, Spotify, Skype, and iTerm 2 + SSH. Occasionally also Photoshop CS5 (I'm not an artist, though).
    Reply
  • ASEdouard - Friday, July 05, 2013 - link

    The 2013 Air surprised me as a, of course light, gaming machine. Granted it's not a new game, but I'm playing Starcraft 2 on high details at an excellent framerate. Pretty fun. Reply
  • KitsuneKnight - Saturday, July 06, 2013 - link

    That reminds me, I actually have Starcraft 2 installed on this MBA. I have to run it fairly near the minimum settings (at the native resolution) to get decent (not amazing) framerates. If the new model is running SC2 at high with rather good FPS, then it's could probably comfortably play many more games than I'd have guessed.

    Of course, with how the typical Windows notebook is spec'ed, they'd likely be no better anyways (shipping garbage GPUs is something every OEM seems to loves to do).
    Reply
  • ananduser - Saturday, July 06, 2013 - link

    The budget shopper, you know, the one the OP made a reference to, has plenty of other better choices that provide more bang for the buck and less profit margin for Apple. The fanboy in you got way to defensive too fast without actually reading my previous post. The budget user won't spend 1000$ on a cramped machine(11") that needs adapters and expensive software licenses -VMWare/Parallels + Windows 8.

    I will make one concession - those better options are not as light and as portable as the 1000$ 11" MBA but they're still plenty portable.
    Reply
  • KitsuneKnight - Saturday, July 06, 2013 - link

    If you're wanting something that's super light and portable, the MacBook Air _IS_ the budget option. You would be very hard pressed to find a machine that trumps its portability and weight, for less (and if you do, you're probably making quite some sacrifices to get it... hell, even if you pay MORE than the Air you're likely getting a garbage trackpad).

    It's funny you say 'less profit margin', because the PC OEMs have continually failed to undercut Apple on the Air. They keep releasing worse machines that are more expensive... and they can only get as close as they've gotten thanks to Intel burning money to motivate them to compete. Apple might have good profit margins, but the consumer isn't paying any "Apple Tax" these days (Apple's process just blows the other OEMs out of the water, sadly enough).

    >needs adapters
    Adapters for what? I have 1 adapter for my Air, a mini-displayport to VGA cable ($20 on monoprice). I got it because I regularly need to use a projector (which most people will never use). And you know what? My machine has proven to be able to work the most reliably with any crappy projector thrown at it.

    >and expensive software licenses -VMWare/Parallels + Windows 8.
    You don't need that. If you do need to run some Windows software, for whatever reason, there's Wine, which works well with most software.

    The reason I have Parallels + Windows 8 + VS2012 was because I needed to do development for the Windows Store, which is a rather rare use case. If I'd used VirtualBox instead of Parallels, I'd have actually spent $0 on the set up, but I'd rather pay a small amount of money for a much better experience, especially when said-experience results in monetary compensation.
    Reply
  • ananduser - Saturday, July 06, 2013 - link

    There are plenty ultraportables that undercut the AIR if undercutting is what you are aiming, so, technically, undercutting the AIR is possible. There will be some compromises but not to the extent that they'd be crap machines as you make them sound.

    Pretty much any ultraportable that is more expensive than the MBA is also better. I think the fanboy in you overreacted again. If Apple's process would've been so good they wouldn't have realeased a 1700$ rmbp that lags while browsing or a 1200$ PRO(?) machine with a 5400rpm drive and a 1280x800 TN screen.

    Intel is paying OEMs for their own gains and not to compete with Apple. Apple refuses any other branding so it cannot tap into the marketing fund. The ultraportable segment is a niche one on any other OEM, because every one of them has broader lineups with more inexpensive gear that gets bought before people arrive at the ultraportable segment. Apple only sells 6 freaking laptop models, the first 2 and the cheapest of the bunch being the MBAs. You practically do not have any choice but to get the MBA for the cheapest entry in OSX land. People don't buy the MBA because it is a thin'n'light ultraportable; they buy it because it is the lowest price point in Apple's portable line up.

    1000$ 11" ultraportables fit for midgets are definitely not budget. It's the budget Apple option for OSX users like you maybe.

    PS:Oh and since this is a pretentious tech site, Apple has also achieved a first. The first next gen machine that is actually slower than the last gen, and in some cases even slower than the one before. Progress indeed.
    Reply
  • shsu - Monday, July 15, 2013 - link

    Hey kitsune, what is your average CPU usage running all that on your base model? Does it ever reach 100%? I have almost same use case: dev tools plus windows vm. Reply
  • ESC2000 - Monday, August 12, 2013 - link

    $1,000+ for the base model is not affordable except to this with a lot of disposable income.

    More importantly, the air isn't a good value, mainly because apple slapped an unnecessarily high price on these as they do with every product, the so-called apple tax. Why does the air cost more than the VAIO even though the air has a crappy display and apple didn't have to pay for the OS license like Sony did? For people who care about affordability, value is really important. Lining apple's pockets doesn't give any value to the buyer.
    Reply
  • iwod - Friday, July 05, 2013 - link

    I have a few things in my mind which i couldn't grasp.

    Surely Apple would have know Samsung had a PCI-E SSD ready in around the same time frame. Why did apple choose to make their own PCI-E SSD and not just uses Samsung's one? I mean after all they both uses the same controller from Samsung.

    Why did Apple decide to use 2x PCI-E 2.0 for its SSD instead of 4x like the one Samsung shown.

    I am looking forward to next year's 14nm Broadwell MBA. Hopefully I wouldn't need a i7 then as the baseline should work just fine. Along with even better SSD and hopefully matured 802.11ac.
    Reply

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