The 2013 MacBook Air: Core i5 vs. Core i7
Post by: bond
Apple typically offers three different CPU upgrades in its portable Macs: the base CPU, one that comes with the upgraded SKU and a third BTO option that's even faster. In the case of the 2013 MacBook Air, Apple only offered two: a standard SKU (Core i5-4250U) and a BTO-only upgrade (Core i7-4650U). As we found in our initial review of the 2013 MacBook Air, the default Core i5 option ranged between substantially slower than last year's model to a hair quicker. The explanation was simple: with a lower base clock (1.3GHz), a lower TDP (15W vs. 17W) and more components sharing that TDP (CPU/GPU/PCH vs. just CPU/GPU), the default Core i5 CPU couldn't always keep up with last year's CPU.
For most users upgrading from an older machine the regression won't matter. The 2013 MBA includes a much faster SSD, better graphics, and better CPU performance compared to older MBAs as well. For those users who are either trying to make an ultraportable more of a workhorse, or who otherwise need more performance than last year's MBA can offer there is one solution. The BTO upgrade increases the base clock to 1.7GHz, adds an extra 1MB of L3 cache and includes much higher turbo frequencies:
Apple 2013 MacBook Air CPU Comparison | |||||
1.3GHz dual-core | 1.7GHz dual-core | ||||
Standard On | 11 & 13-inch MBA | Optional on Both | |||
Intel Model | Core i5-4250U | Core i7-4650U | |||
Base Clock Speed | 1.3GHz | 1.7GHz | |||
Max SC Turbo | 2.6GHz | 3.3GHz | |||
Max DC Turbo | 2.3GHz | 2.9GHz | |||
L3 Cache | 3MB | 4MB | |||
TSX-NI | No | Yes | |||
TXT | No | Yes | |||
AES-NI | Yes | Yes | |||
VT-x/VT-x EPT | Yes | Yes | |||
VT-d | Yes | Yes | |||
TDP | 15W | 15W | |||
Processor Graphics | Intel HD 5000 | Intel HD 5000 | |||
GPU Clock (Base/Max) | 200/1000MHz | 200/1100MHz |
Careful binning makes all of this possible within the same TDP and cooling solution as the standard configuration.
The CPU upgrade comes at a fairly reasonable cost: $150 regardless of configuration. The max clocks increase by almost 30%, as does the size of the L3 cache. The obvious questions are how all of this impacts performance, battery life and thermals. Finally equipped with a 13-inch MBA with the i7-4650U upgrade, I can now answer those questions. The two systems are configured almost identically, although the i7-4650U configuration includes 8GB of memory instead of 4GB. Thankfully none of my tests show substantial scaling with memory capacity beyond 4GB so that shouldn't be a huge deal. Both SSDs are the same Samsung PCIe based solution. Let's start with performance.
Boot Performance
Although boot performance doesn't change substantially with the faster CPU, I did notice that the new system was posting better boot times than what I had previously recorded. My guess is all of my SSD torturing hadn't been TRIMed away before I presented the previous results. Either way the new MBA seems capable of a sub-12 second boot time. I even recorded some boots as low as 10.8 seconds. That PCIe SSD comes in handy for sure.
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.
Battery Life
I ran the i7-4650U based 13-inch MBA through the same suite of battery life tests as the un-upgraded base model. 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. The other thing I noticed while testing these two machines is that although both were equipped with the exact same Samsung panel, their brightness curves were slightly different. At the same brightness setting under OS X (or Windows 8), both panels responded differently. Both were capable of similar max brightness values but it's clear that either Apple is doing some calibration here that's panel lot specific or there's a significant variance in how these panel/backlight/electronics combinations respond (or both).
Apple from design and software all the way down to the purchasing experience. The 2013 MacBook Air offers only two choices of CPUs, and honestly for the vast majority of the population, that's all you really need. The default Core i5 1.3GHz (4250U) delivers the best overall battery life regardless of workload. Its performance is often somewhere in between a 2011 and 2012 MacBook Air depending on workload, although in some cases it's possible to see equivalent performance to an upgraded 2012 MBA. If you need more performance however, the 1.7GHz Core i7 upgrade (4650U) delivers. In most situations you get more than a 20% increase in performance, bringing the platform up to somewhere in between last year's 1.7GHz Core i5 and 2.0GHz Core i7 options. Once again, with the right workload you could even see performance as much as 20% better than a 2GHz Core i7 from last year. Although I didn't publish any results here, GPU performance seemed roughly unchanged compared to the Core i5 option.