Testing Turbo

The original MacBook Air had a 1.86GHz Core 2 Duo that pretty much never ran at 1.86GHz. Instead the chip typically ran at or below 1.4GHz, allowing the second generation 11-inch MacBook Air to outperform it. Since then I've been quite concerned about Apple playing clock speed games with its systems, but thankfully since then we haven't seen any similar issues.

Letting my guard down wasn't something I was interested in doing so I went about verifying Turbo on the new Air. First a quick refresher about what Turbo Boost does on Intel CPUs. All CPUs are designed to certain power and thermal limits. Those limits are shared with manufacturers who then spec power supplies, cooling systems and cases that can handle those CPUs. In the old days this was pretty simple, you had a single core and a single thermal design point (TDP) that you had to hit. Then multicore CPUs came around and made things more complicated. TDPs couldn't go up (laptops weren't going to double in size) but you now had multiple cores sharing the same TDP, so each core had to dissipate less heat. This meant that your dual-core CPU would run each core at a lower frequency than your single-core CPU, and your quad-core CPU would run at an even lower speed. When using all of your cores the tradeoff was worth it, but when only one or two were in use, it wasn't. We needed a way of shutting down cores to free up TDP and dynamically adjusting clock speed.

  Single Core Dual Core Quad Core
TDP
Tradeoff

Nehalem came along and introduced power gating to x86 CPUs. Unused cores could be power gated, effectively shutting them off, and the resultant available TDP could be used by turbo boosting active cores. All subsequent Intel architectures have supported Intel Turbo Boost, a feature we investigated extensively in our 2011 MacBook Pro review earlier this year.

Lion got rid of the 32-bit kernel so my previous trick to run MSR Tools to measure clock speed stopped working. I needed an alternative.

I offer you three pieces of evidence that support the theory that Apple is not artificially limiting clock speed on any of the MacBook Air systems I've tested.

The first is output from Lion's boot process, note the line that begins with AppleIntelCPUPowerManagement:

The screenshot above is from a 1.6GHz Core i5 on an 11-inch MacBook Air. This chip runs at 1.6GHz by default but can turbo up to 2GHz with two cores active and 2.3GHz with one core active. The line above lists turbo ratios 0047. This four digit number shows turbo ratios for 4C/3C/2C/1C active, with each digit corresponding to a different max turbo ratio. Each value is in hex to represent ratios above 9 with a single digit.

Take the example above: 0047. The first two numbers are 0s because the chip doesn't have more than two cores and thus doesn't support any turbo ratios when 4 or 3 cores are active. The third number tells us the maximum turbo boost with two cores active: 4. That's 4 bins, where each bin is 100MHz, or 400MHz above the stock 1.6GHz operating frequency (2.0GHz).

The fourth number gives us max turbo when only a single core is active: 7. Seven bins is 700MHz, which on top of the 1.6GHz base frequency gives us 2.3GHz.

What about the 1.7GHz Core i5 in the 13-inch MacBook Air? Its turbo ratios look like this:

The first two digits are 0 again since this is a dual-core processor. The third digit is a 7, representing a 700MHz max turbo boost with two cores active. The fourth digit is an A, which is 10 when you convert from hex to base 10, or a maximum 1GHz turbo boost with one core active. If you had a 1.8GHz Core i7 you'd see the string 008B.

The second bit of evidence supporting that Apple isn't mucking with clock speeds comes via Cinebench. The 2.0GHz Core i7 in the 15-inch MacBook Pro can turbo up to 2.9GHz with only one core active. Cinebench has a great single threaded test that ensures only one core is active. The chip in the 13-inch MacBook Air should be able to hit 2.7GHz with one core active. We already know the CPUs in the 2011 MacBook Pro have fully functioning turbo ratios based on our earlier investigations. While the i7 in the MacBook Pro has a larger L3 cache, we should still see around a 7% performance advantage in single threaded Cinebench for the 2011 MacBook Pro vs. the 13-inch MacBook Air (2.9GHz vs. 2.7GHz) if the Air is properly turboing up:

Clock Speed Scaling Comparison
  15-inch MacBook Pro (Core i7 2.0GHz) 13-inch MacBook Air (Core i5 1.7GHz) 11-inch MacBook Air (Core i5 1.6GHz)
Cinebench R10 - Single Thread 4060 3770 3154

The 2011 MacBook Pro is 7.7% faster in Cinebench's single threaded test. Perfect. We can't do the same comparison with multithreaded performance because the i7 in the MacBook Pro has twice the number of cores/threads as what's in the Air, but by this point I'm pretty confident that Apple is allowing these chips to turbo up.

If you need one more piece of evidence I present to you behavior of the 2011 11 and 13-inch MacBook Air under Windows 7, where we do have good turbo monitoring tools courtesy of Intel.


13-inch MacBook Air, Core i5 1.7GHz, max single-core turbo

Both systems had no problems hitting their specified turbo ratios, we are not artificially constrained here at all. In fact, the 11-inch MacBook Air was able to even exceed its dual-core turbo ratio by a small amount during some of my testing:


11-inch MacBook Air, Core i5 1.6GHz, temporarily exceeding max dual-core turbo

In the example above the 1.6GHz Core i5 is running a multithreaded workload, Cinebench once again. Both of its cores are pegged at 100% and it is able to turbo up to 2.0GHz, its max turbo frequency with two active cores. For a short period of time during the render however, we actually see numbers above 2GHz.


11-inch MacBook Air, Core i5 1.6GHz, max single-core turbo

This is a feature of Sandy Bridge. If your chip is idle then all of sudden gets a heavy workload it'll try to turbo up to its max ratio. The chip itself will take time to actually hit its max operating temperature though, it doesn't simply jump up to max TDP just because it's running at a higher frequency. During this TDP ramp, Sandy Bridge actually allows the chip to run even faster than its max turbo frequency for a short duration. This overboost gives you a little extra oomph in performance without ever violating the chip's TDP.

The CPUs A New Thunderbolt Implementation
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  • mschira - Friday, July 29, 2011 - link

    well you are not thinking apple enough for this. saving 100$ is no reason accepting a mess on your table...
    besides who sais you need to toss the display when the gfx is outdated? what's keeping you from connecting the display via A dedicated GPU box once the internal GPU becomes to slow?
    M.
    Reply
  • darwinosx - Saturday, July 30, 2011 - link

    In the display? So when the card is obsolete you get a new display? How about outside the laptop and the display but in between both. Reply
  • AJ Driver - Thursday, July 28, 2011 - link

    Thanks Anand :)

    The irony for me reading your review and others around the internet is the comparison in performance between these MBAs and other models (both new and old).

    My personal laptop is a 5, going on 6, year old Macbook and for what I use it for it's more than good enough. It's pretty incredible the way technology is advancing, and if the current pc stays the course I wait as long as I can before taking the leap.

    It's hard to deny the urge to upgrade though!
    Reply
  • Marand - Thursday, July 28, 2011 - link

    Well I thought I was finally clear that I was going to buy a MacBook Pro 13 but now I am not so sure.

    I mainly use my laptop for software development. My previous one was MacBook pro 2007 model which served me well.

    My issue with the pro line is that because of their weight, it just ends up becoming a desktop that I don't take with with me and always plugged into a large screen.

    But having the ability to put 8 gigs of ram and update the hard drive on the new MBPro is a nice option to have.

    I really hoped that apple was going to release MBAs with 8 gigs of ram at least and even if you can't upgrade the hard drive (although I heard you can with prev gen from OWC) you could always plug in bigger drives through thunderbolt if and when drives comes out.

    The big selling factor for the MBA for me was the weight. I figured it would be super easy to take with me where and when I want without treating it like a desktop.

    I know the MBAs are not targeted to developers. Ut I know plenty who were hoping for "more" from MBA

    So now I have a tough choice because the MBpro 15 with quad core and the hi res anti glare screen is packed with power even if it's heavier and likely has more long term capability.

    Oh well, can't have it all I guess...
    Reply
  • A5 - Thursday, July 28, 2011 - link

    If the 15" MBP is too heavy for you, you may want to get a more supportive bag or something. I would've killed for a 5.6lb 15" laptop 8 years ago :P Reply
  • Uritziel - Thursday, July 28, 2011 - link

    Remember, you're not supposed to carry it around attached to your ear lobe :)

    I carried a 17-inch DTR laptop that weighed about 10 lbs. around campus for a year, so 5.6 lbs. sounds light to me too. I consider battery life as the main metric of portability after weight hits that 5 lbs. mark.
    Reply
  • crimson117 - Thursday, July 28, 2011 - link

    >The only exception is if you're just going to spend your time doing very basic tasks on the machine and plan on upgrading again in a year or two. If that's the case save your money and enjoy a 4GB version with Ivy Bridge next year.

    People who can afford a new MacBook Air every time a new one comes out aren't going to be worried about saving their money on a ram upgrade :)
    Reply
  • steven75 - Thursday, July 28, 2011 - link

    Apple hardware has incredible resale value, so it's actually easier to do than you think. Reply
  • Uritziel - Thursday, July 28, 2011 - link

    Don't forget that if you buy a laptop for $2000 today and sell it at that great Apple resale value of say $1000 (numbers are random) in a few years when you're ready to upgrade, the laptop did NOT cost $1000 in the end.

    Adding on the effort required to resell it, that rationale makes a lot less sense than some people claim.
    Reply
  • Rasterman - Wednesday, August 03, 2011 - link

    I'm looking at upgrading right now so I looked up the resale value of mine. I bought a new MB for $1150 in 2008, used prices are $500-$800 right now, I expect to sell mine north of $700 since it is in brand new condition as I only used it as a dev machine when porting and it has seen very little use. So my expected resale value is 61% after 3 years which is pretty damn good IMO. Reply

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