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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  • Paapaa125 - Thursday, July 04, 2013 - link

    You lack one kind of test:

    Test light/medium workload BUT equal the amount of work the units do. For example encode 2 hours of movies, reload the webpage 200 times and see the result. The workload is identical (i7 does it faster) and how does it affect the battery life?

    Of course i7 uses more juice in heavy workload but it also accomplishes more. This is not necessarily fair.
    Reply
  • lukarak - Thursday, July 04, 2013 - link

    "What's important to note about all of these tests is that the amount of work done per cycle of the test doesn't vary based on performance. There's enough idle time baked in to make sure that the Core i7 based 13-inch MBA isn't artifically penalized by having to do more work than the i5 model simply because it's faster. "

    You obviously missed this?
    Reply
  • Paapaa125 - Thursday, July 04, 2013 - link

    Yes, I missed that. Very important correction, thanks! So i5 does more with the same battery life, just slower. Reply
  • ananduser - Thursday, July 04, 2013 - link

    It doesn't do more. It does the same only slower. The i7 finishes the job faster and you pay a battery penalty for that, obviously. Reply
  • Paapaa125 - Friday, July 05, 2013 - link

    Ok, let's sort this out. I was under impression that both computer do the same amount of work in same time? So if both ran a test for one hour, they accomplished the same thing?

    And as i5 can last longer than i7 in heavy load, it also can do more work, right?
    Reply
  • ananduser - Friday, July 05, 2013 - link

    Both kits are tested with the same amount of work. The i7 draws more power obviously but it also finishes its tasks faster than the i5. The i5 lasts longer but it also takes longer to finish the heavy load because it is slower. Reply
  • wallysb01 - Saturday, July 06, 2013 - link

    Which shouldn't be too surprising, speed increases per Watt are not linear. I would have guessed in the sub 2.0GHz base clock rate range that it might be closer to linear than this showed though. Reply
  • Sugardaddy - Thursday, July 04, 2013 - link

    That was a very good and informative article, thank you! Now if only we could get a review of the 28W Haswell chips! Reply
  • dineshramdin - Thursday, July 04, 2013 - link

    It costs me around 140 bucks to upgrade my Core i7 CPU, which was pretty reasonable. However, i7-4650U includes a 8GB memory and the other one got one of 4GB, I guess Reply
  • Abelard - Thursday, July 04, 2013 - link

    Those battery life figures are eye-poppingly good. Reply

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