The CPU Selection

Like the 2011 MacBook Pro upgrade, the iMac gets the Sandy Bridge treatment. Given the heat-dissipating volume of the iMac's all-in-one form factor, Apple only offers a quad-core CPU throughout the lineup. Just a couple of years ago quad-core CPUs weren't mainstream but I believe today it's safe to say that four cores are going to be the standard going forward. We'll see two cores stick around for small form factors and budget systems but everything else is moving to four. Intel is still toying with the idea of 6-cores for the high end but I'd say there's likely even less traction for 6-cores today than there was for 4-cores a few years ago. While high end desktop users could easily make the argument for 4 cores, it's much more difficult to do the same for 6 unless you're building more of a workstation.

I've described Sandy Bridge several times in the past so I won't belabor the advantages here, but the advent of aggressive turbo modes basically rids the OEM of any reason to make a trade off between more cores and higher clock speeds. Sandy Bridge offers you the best of all worlds - high clock speeds in lightly threaded applications or more cores when you need them.

The iMac is less TDP constrained than the MacBook Pro so we get higher base clock speeds to begin with. In fact, Apple opts for lower priced desktop CPUs than the mobile chips. They are clocked higher and put out more heat but they do help Apple maintain that healthy profit margin. Take a look at Intel's price list:

2011 iMac Comparison
Mobile CPU Price Desktop CPU Price
Intel Core i7-2820QM (2.3GHz quad-core) $568 Intel Core i7-2600 (3.4GHz quad-core) $294
Intel Core i7-2720QM (2.2GHz quad-core) $378 Intel Core i5-2400 (3.1GHz quad-core) $184

The $2199 15-inch MacBook Pro comes with a 2.2GHz mobile Core i7, the 2720QM to be specific - the CPU itself costs $378. The $1999 27-inch iMac comes with a 3.1GHz desktop Core i5-2400, the CPU here is priced at a much lower $184. The most expensive CPU you can buy in the 27-inch iMac is a Core i7 2600, which Intel charges $294 in 1,000 unit quantities. There's physically more hardware in the iMac, but using desktop CPUs is a no brainer for Apple here.

The CPU lineup is a bit strange:

2011 iMac Comparison
iMac Model $1199 21.5-inch $1499 21.5-inch $1699 27-inch $1999 27-inch
Base CPU Intel Core i5-2400S (2.5GHz quad-core) Intel Core i5 2500S (2.7GHz quad-core) Intel Core i5 2500S (2.7GHz quad-core) Intel Core i5 2400 (3.1GHz quad-core)
CPU Upgrade Offered None Intel Core i7 2600S (2.8GHz quad-core) None Intel Core i7-2600 (3.4GHz quad-core)

The 21.5-inch iMac comes with a Core i5-2400S or 2500S by default. You can upgrade to the Core i7-2600S but only if you buy the $1499 system. Based on iFixit's teardown you should be able to do a CPU upgrade on your own since these are just standard socketed parts. Note that the 21.5-inch iMac only uses 65W TDP CPUs, the S suffix drops base clock speed a bit in exchange for lowering the max TDP from 95W down to 65W. Remember how turbo works, with lots of cores sharing a low TDP the base clock might be low but that means that you've got more room to turbo up when you start powering cores down. Despite the 2.5GHz base clock speed, the Core i5-2400S can turbo up to 3.3GHz with a single core active. The 2500S reaches a staggering 3.7GHz at max turbo.

The 27-inch iMac starts with a Core i7-2500S, but the upgraded model moves to a 95W Core i5-2400 clocked at 3.1GHz. Believe it or not but the i5-2400 can only turbo up to 3.4GHz. Running single threaded applications, the cheaper iMacs will actually be a little faster. There's also a Core i7 upgrade offered here, but again only for the more expensive iMac: Apple will sell you a Core i7-2600 for an additional $200.

Apple 2011 iMac CPU Comparison
  2.5GHz Core i5 2.7GHz Core i5 2.8GHz Core i7 3.1GHz Core i5 3.4GHz Core i7
Intel Model Core i5-2400S Core i5-2500S Core i7-2600S Core i5-2400 Core i7-2600
Base Clock Speed 2.5GHz 2.7GHz 2.8GHz 3.1GHz 3.4GHz
Max SC Turbo 3.3GHz 3.7GHz 3.8GHz 3.4GHz 3.8GHz
Max DC Turbo 3.2GHz 3.6GHz 3.7GHz 3.3GHz 3.7GHz
Max TC Turbo 2.8GHz 3.2GHz 3.3GHz 3.3GHz 3.6GHz
Max QC Turbo 2.6GHz 2.8GHz 2.9GHz 3.2GHz 3.5GHz
L3 Cache 6MB 6MB 8MB 6MB 8MB
Cores/Threads 4 / 4 4 / 4 4 / 8 4 / 4 4 / 8
AES-NI Yes Yes Yes Yes Yes
VT-x Yes Yes Yes Yes Yes
VT-d Yes Yes Yes Yes Yes
TDP 65W 65W 65W 95W 95W

Now let's talk architecture. All of the Core i5s come with a 6MB L3 cache, while the upgraded i7s come with an 8MB L3. The even bigger difference? None of the Core i5s come with Hyper Threading enabled - they are four core, four thread chips. Only the upgraded Core i7s have HT enabled, giving them a total of eight threads. By comparison, all 15-inch MacBook Pros are mobile Core i7s with 8MB of L3 cache, four cores and eight threads. In other words, given the right workload, a high end 15-inch MacBook Pro could actually outrun one of these iMacs.

Hyper Threading only really matters with heavily threaded workloads (video encoding, offline 3D rendering) so I doubt most users would notice a difference, but it's still a shame that the iMac can't claim total superiority over the MacBook Pro.

I turned to MSR Tools once again to verify turbo operation. Running a single threaded instance of Cinebench the tools reported a maximum clock frequency of 3.3GHz. Assuming MSR Tools itself is keeping a second core awake, turboing up to 3.3GHz makes sense. I have no reason to believe that Apple is artificially limiting max turbo speeds, at least on the 27-inch 3.1GHz Core i5 model. Given how much room there is inside the iMac to dissipate heat, I don't see any reason for Apple to limit clock speeds here.

The quad-core CPU idles at 1.6GHz under OS X

Quick Sync is of course one of Sandy Bridge's biggest features and once again it goes relatively unused within the iMac. FaceTime HD supports it but iMovie, which ships with the system, has yet to be updated to take advantage of Quick Sync. If you want to upgrade to Sandy Bridge in order to get better video transcoding performance courtesy of its hardware decode/encode engines, I'd recommend waiting until Apple actually updates its software before making the move to Sandy Bridge on OS X.

Two Models Intel's Z68 Chipset, Thunderbolt & Display IO
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  • KoolAidMan1 - Friday, May 27, 2011 - link

    "The example given by headbox is a prime example."

    I can't think of any Mac user who would really try this. I mean, I plug my gaming PC into my 27" iMac as a primary display through the mini-DP port, and I figure I'm in an extreme minority of users. People who plug consoles and BR players and who would need a converter box is be an even tinier number of users.

    Don't get me wrong, I'm annoyed that the 2011 iMac has new Target Display Mode requirements, but the limitations of prior models in terms of using set-top boxes (consoles, Blu Ray players) isn't statistically a big enough number to get bent out of shape over, IMHO.
  • Tros - Friday, May 27, 2011 - link

    "How are you going to upgrade the motherboard on a proprietary and overpriced all in one?"

    This is as much of a criticism on laptops, tablets, smartphones, as all-in-one units. And guess what: That proprietary junk has been of great value to a lot of people, especially if it has an aesthetic appeal.
  • Penti - Friday, May 27, 2011 - link

    While certainly great for what they for probably, it's definitively not the same thing using 23" 1920x1080 120Hz TN-screens with Nvidia 3D Vision as with a good IPS or PVA screen with proper viewing angles and for mostly other uses then gaming. Of course you need your PC to your triple display gaming machine and a strong GPU too.
  • rubaiyat - Wednesday, September 7, 2011 - link

    Do you actually use your computer for much besides gaming?

    I'd rather have an excellent monitor than some dodgy and essentially useless 3D 'feature'.

    But that separates consumers in all areas. Those who will see a movie because it has (very loud) surround sound and pseudo 3D with lots of explosions, and those who will see a movie because it actually IS a good movie.

    My iMac27 has a brilliantly sharp and accurate 27" 2540x1920 display, that thankfully is not 3D nor runs generally awful Windows grade video.
  • nafhan - Friday, May 27, 2011 - link

    I'm fine spending more on a monitor than a desktop... However, I'll usually go through two or three desktop hardware upgrades before I replace my monitor.
  • fitten - Friday, May 27, 2011 - link

    And when the monitor is the majority of the cost of the system, it makes sense to reuse it for future upgrades. Monitor technology seems to evolve slower than the rest of the system so barring some major changes, keeping it for several upgrades won't 'set you behind' any. So, reusing a monitor is an extremely cost efficient technique to keep your computer 'modern'.
  • Spivonious - Friday, May 27, 2011 - link

    Exactly. I've had the same NEC (Mitsubishi tube) CRT on my desk for 15 years. The picture is still fantastic and blows any TN-panel LCD out of the water.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Good thing they use the best IPS panels in these things. :)
  • Guspaz - Friday, May 27, 2011 - link

    Colour-reproduction wise, perhaps, although that depends on the backlight. In terms of detail and a crisp image, even a cheap TN panel will destroy an old CRT, and IPS panels will match or surpass such a CRT.

    I'm reminded of a friend of mine, who for years (until perhaps 2-3 years ago) insisted that his old CRT monitor was fine, despite the fact that it was so out of focus that 14 point text was unreadable. He finally relented and upgraded when we proved to him that he was in denial when we realize that the reason he didn't have trouble reading on the monitor was because he increased the text size by 200-300% when he used the monitor. Now, I'm not suggesting that your monitor is out of focus, a good CRT monitor can have excellent sharp detail. But even the best of them comes nowhere close to a half decent LCD.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Not at all. Sell the iMac and get a better monitor that comes in the next update, it happens every 2-3 years. Resale value is also high.

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