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
POST A COMMENT

127 Comments

View All Comments

  • TheinsanegamerN - Wednesday, July 10, 2013 - link

    was there any increase in graphical performance vs the i5? Reply
  • antonio22m - Wednesday, August 14, 2013 - link

    Macbook Air is undoubtedly a very good laptop.
    Price and lack of optical drive can affect a large number of users whose decision during the judgment can be negative so that they can decide to choose another manufacturer.
    Air is perfect and the best "second computer" that you can wish for.
    His task was not to be the main and only computer we can possess.
    If you want excellent laptop computer that will be able to carry it with you wherever you go, the Air is an excellent choice for perfectly reasonable size and more pronounced weight that barely exceeds one kilo.
    Take a look at this comparison at <a href="http://www.squidoo.com/apple-macbook-air-133"... MacBook Air 13.3</a> and You will see comparison to the another Apple laptops.Anyone considering purchasing this laptop needs to see the information in this chart.
    Reply
  • Mr_Data - Thursday, August 15, 2013 - link

    I more powerful CPU would finish a task faster than a slower CPU. I made so calculations and for medium and heavy loads the i7 has a lower battery life with 12.65% and 15.37% than the battery life of the i5. But the performance increase for the i7 for heavy processing (Adobe Lightroom, compiler test) is between 17.21 and 23.25% over the performance of the i5. Either way, starting from a full battery, you'll finish more tasks and faster with i7 than with the i5. Reply
  • Kestryn - Saturday, November 09, 2013 - link

    Just want to say, "thank you" for your well-written and organized feature on the MBA. In today's environment of waning journalistic excellence, it is always refreshing to find a good writer. Not that you've asked for advice, but if you have a taste for investigative adventure, perhaps you should link up with one of the new philanthropy-spawned internet "newspaper" outfits. Cheers, and keep writing! Reply
  • tetsuk - Friday, November 15, 2013 - link

    Can anyone with the 1.7GHz model comment on the temperature? I have actually suffered from low heat burns due to the constant heat generated by a Macbook pro in the past. The temps in this article seem to go extremely high under medium-to-high load. Reply
  • nerd1 - Wednesday, May 21, 2014 - link

    I just checked the benchmark result of retina MBP 13 - and SP3 actually slightly beats it!

    I'm not sure why anand did not put rMBP 13 for comparison (a more fair comparison than quadcore rMBP 15).
    Reply
  • omaudio - Wednesday, July 23, 2014 - link

    Hi,
    We bought a 2014 MBAir from MacMall a few days ago- it was the i7with 8GB and 256GB SSD. It randomly shut off and when we went to Apple to buy one direct from them they aid ours was likely one that was defective, "customized" and sold on 2nd tier market. I then noticed the i7 model w 8GB has been removed from their site. What gives? Design flaw? The components are soldered so not sure why they would say "customized".
    Reply

Log in

Don't have an account? Sign up now