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 - Sunday, May 29, 2011 - link

    iMacs don't use mobile CPUs. My gaming PC and my 2009 iMac have the exact same CPU, a Core i7-860
  • boop - Saturday, May 28, 2011 - link

    anand, i used to have the same dilemma you face regarding the syncing of files across multiple machines. i even did the same thing as you and tried to use an macbook pro as a desktop replacement. it didn't work out so well. a notebook that's powerful enough to be a desktop replacement isn't portable enough -- and a notebook that's particularly portable just isn't powerful enough to be a desktop replacement.

    The solution I found was to use google docs and other related online services to store all my work files. as you know you can access the files on any machine, and even be logged in on multiple machines simultaneously; documents can be shared with others for collaborative work; and it also solves the problem of data back ups and laptop theft since nothing is locally stored. my current Mac setup is a 13" macbook pro and the 2011 21.5" base model imac and with all my work stored in the cloud i've finally found an optimal solution to my workflow woes. hope this helps!
  • ananduser - Saturday, May 28, 2011 - link

    Apple packages tech real well. Good display, reasonable PC, all'n'all a good looking AIO that especially does not compromise on the display side of things. Glad I am not in need for the OS or the hardware flexibility of the laptop combined with the mobility of the desktop.
    Even if Apple gets so much marketing coverage everywhere I am still glad that I see my own kind keeping criticism alive.
  • Hrel - Saturday, May 28, 2011 - link

    That display is worth 400 bucks tops; it's insane to me that anyone would pay a grand for a display barely above 1080p. I can get a good ISP 27" 1080p display for 300, so that thing is worth 400 tops; which makes that whole system 300 dollars too expensive to even consider. Then there's all the laptop parts and non-upgradeability and Appleness to ruin the experience.
  • Kristian Vättö - Saturday, May 28, 2011 - link

    Find a similar display for 400$ then. It's ridiculous to argue that the display can't be worth more than 400$ when similar displays cost 1000$. If you don't think it is worth it, then that is your thing. It doesn't change the fact that the display is worth ~1000$.

    I also doubt that you can find a good 27" IPS display for less than 300$. Most of the good ones are either ~22" at that price range or you have to pay more.
  • donnyg - Saturday, May 28, 2011 - link

    You can get a Hazro, which is literally the same panel as the current Apple CInema Displays minus the aluminum casing, for around $500 USD.

    Shipping will make it cost considerably more however.
  • KoolAidMan1 - Sunday, May 29, 2011 - link

    There is a massive different in pixel density between a 27" display at 2560x1440 and 1920x1080, HUGE. Then there's the fact that your display is using a crap TN panel instead of an IPS, isn't LED backlit, etc etc.

    A good 24" IPS display from HP or Dell is going to cost $400-$500, and a 27" with the same panel from Dell or NEC is going to cost $1100-$1400.

    For a site with a "techie" readership, it is amazing how little some people know about tech, or how much they are willing to excuse purely based on price. Yeah you can save money on a cheaper display, but it is going to be totally inferior in every way except for price. You get what you pay for.
  • MadMacMan - Saturday, May 28, 2011 - link

    That's exactly right. DO YOUR RESEARCH, people. There is NO equivalent in the 27" category of HIGH-END displays and by that I mean not part but ALL of the following:

    LED back lighting (No CCFL bulb, thank you), in-plane switching (IPS); not your average (READ: average, as in mid-range, as in lame, as in cheap, as in what a lot of you might buy and post BS about how you're not getting ripped off by Apple because you're so much smarter; you know who you are). Finally, and perhaps even more problematic to find is its high RESOLUTION! 2560x1440. That's 78% more real estate than you get on a standard 1920x1080 display.

    I dare anybody to Google me up a 26"-29" monitor with the aforementioned specs, but ALL of them, not one or two. Now go to Best Buy and buy yourself a cheap HP or order up an Acer and make sure you don't forget to rub it in. ;-)
  • donnyg - Saturday, May 28, 2011 - link

    IPS monitor: Check
    2560x1440 resolution: Check
    LED backlighting: Check
    Price: 569 AUD for the the one with extra inputs/scalar,
  • Kristian Vättö - Sunday, May 29, 2011 - link

    I don't know where you got your 500$ as 443£ is equal to 731$. That seems to be without shipping. Cheaper? Sure, but not that much.

    You can always save a few bucks if you do some shopping and build it yourself. The iMac, or any Apple product, has never tried to be the cheapest option or best bang per performance on the market. I would still say the new iMac is great value. I already mentioned why an OEM PC and a homebuilt one are not always comparable, especially when we get into an Average Joe level. Hardcore gamers will never be satisfied with Apple's offerings but to be honest, I don't get it why they complain then. Nobody is trying to sell them an iMac.

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