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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  • Esko747 - Thursday, July 04, 2013 - link

    Thought: Would using an efficient laptop cooler increase the performance at noticeable rate? The chassis in these are after all used as a heatsink, and the main thing stopping the processor from using higher clocks for longer periods of time is heat. Reply
  • IntelUser2000 - Thursday, July 04, 2013 - link

    That depends on whether cooling solution in the Macbook Air is good enough for the TDP or not.

    Yes, if the system is substandard, minor to zero if its not. Haswell uses similar Turbo system to Sandy/Ivy Bridge where it has a "guaranteed" Turbo and higher "opportunistic" Turbo, which the latter would turn off after a certain period regardless of how cool its running(to keep it at 15W power use averaged out over longer periods).

    I'm guessing the 2.8GHz reached for the Core i7 in the Multi-threaded suite is the "opportunistic" Turbo, and in infinite time periods with sufficient cooling, it'll settle at 2.4-2.5GHz.
    Reply
  • SteveKen - Monday, July 08, 2013 - link

    Love my job, since I've been bringing in $82h… I sit at home, music playing while I work in front of my new iMac that I got now that I'm making it online. (Home more information)
    http://goo.gl/FdFf2
    Reply
  • FwFred - Thursday, July 04, 2013 - link

    This reminds me of the Nexus 4 freezer test. Anyone willing to stick their MBA in a freezer??? All in the name of science of course. Reply
  • mavere - Thursday, July 04, 2013 - link

    Based on my past experience, the Macbooks don't thermal throttle until ~100C, so within the context of this article, I doubt there'd be much difference.

    That might change with the fan gets dusty, though. One can also reapply the thermal paste inside the machine for a (frustratingly) consistent 10-15C drop and lower fan noise.
    Reply
  • dsumanik - Thursday, July 04, 2013 - link

    "Oh man, the Core i7 upgraded seriously fixes everything."

    -anand lai shimpi

    Direct acknowledgement the initial 2013 air review left you with a sense of disappointment, and sub par performance. Yet you read that review and its all roses, praises and recommendations, furthermore followed by a direct advertising "best mac laptops - June 2013" article, which recommends purchasing the same laptop.

    I mean after all if the 2013 MBA was so great there would be nothing to "fix"

    RIGHT?

    RIIIIIGHT?????

    Samsung made a new PCIE SSD and intel released a new processor.
    Apple put it in the same package. That is ZERO ZERO ZERO innovation.
    They are still recycling products from the JOBS ERA thats why the stock is going to continue to fall.

    You want them to start innovating again Anand?

    Rip apple a new asshole when they re-release this same junk!!!!

    I have been coming to this site for over a decade.

    You, anand, personally, directly, have lost your impartial journalism and professionalism.

    is Tim Stevens on your speed dial or what?

    What possible motive could you have for ignoring the facts and promoting their products blindly like this?

    There is only one explanation......like many other media blogs, AT has substantially invested and now lost money in apple stock, or are directly, or indirectly getting kickbacks and are now trying to stop the bleeding.

    "Oh man, the Core i7 upgraded seriously fixes everything."

    EXCEPT

    - same design as last year- stethoscope when plugged in, switch the TB PORT!!! NO BRAINER!!!

    - in 2013 a "premium apple experience" delivers 1366 x 768 resolution.. I mean come on 1998 era 1600x1200 would have been at least SOMETHING LOL..and absolutely technologically possible.
    I mean come on you have double the battery life now!!!!!!!

    - no 16gb ram option???? so cheap and easy to do. But nah, peeps will still buy it. Thaty is exactly how the board meeting on that one went.

    - no LTE or modem support, you should buy an iPhone for that.

    - there are wifi problems galore with the new model, and wifi limitations within OSX itself....don't believe me? .... you reported it in your last article but any MBA owners running boot camp have known this for over a year now....you need to transfer a big file? Boot into windows and cut the time in EXACTLY half.

    Eff this shit im out, this site's owner is straight up corrupt...at least he got burned on apple stock for it.

    Pay backs a bitch huh.
    Reply
  • JKflipflop98 - Thursday, July 04, 2013 - link

    People like you are a cancer on this Earth. I hope you finally succumb to your syphilis infection. Good day, sir. Reply
  • KoolAidMan1 - Friday, July 05, 2013 - link

    Apple haters are more annoying and idiotic than any Apple fanboy, which is quite an achievement. People like this are amazing Reply
  • iwod - Friday, July 05, 2013 - link

    Yes, Apple Fanboy and Haters, the irony. The truth is, Anyone, absolutely anyone who work within the industry would have had at least a little bit of respect to Apple. Be it Intel, AMD, or heck even Samsung or Google. And by ALL standards Apple has consistently put fourth one of the best hardware.
    So in that sense Apple Fanboy are at least better, because even if they know jack about Apple it still have a product that is worth talking about.
    And Apple haters, are like you said annoying as hell as most of the time they dont have a clue about what innovation actually means and literally knows nothing about design, engineering and tech. And what's even worst is that they "think" they know a lot. And They just hate because they hate.
    And back to those work within the industry and why they actually have a little respect for Apple? Because they actually know their stuff. Not just clueless mind bogging trolls who goes on the internet everyday reading mostly incorrect and false statement and pretend they know a lot.
    Reply
  • dsumanik - Friday, July 05, 2013 - link

    nothing to do with apple love or hate...I've pointed out that anand wrote a positive article about the MBA, then in his own words, revealed deep down inside he felt it needed "fixing"

    He then went on to write a second article further promoting this product, specifically promoting it.

    Connect the dots folks, stop changing the subject to apple vs microsoft and goole, i know your hamster brains are trained to do that...its reflex now....but my point was ananda bias, and then advertising of a product he was surprised to find slow, and at best on par with last years model... and still unchanged as a platform and product as a whole.

    dont hate me cuz it has an apple logo on it.
    Reply

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