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
Comments Locked


View All Comments

  • joe_dude - Saturday, May 28, 2011 - link

    But the thing's essentially a 3-year old PC. Why the heck would I want to pay more to downgrade?

    Of course Macs hold their resell value... it's not for gaming, so even a 5 year old Mac seem pretty fast using regular apps.

    Even if it can be used as just a monitor, who the heck wants the extra dead weight? All-in-ones will always be a compromise. The point of having a desktop PC is *not* to compromise. Otherwise, a laptop can do the job.
  • Kristian Vättö - Saturday, May 28, 2011 - link

    Good luck finding a Sandy Bridge CPU and AMD 6000-series graphics in a 3-year-old PC.

    A LOT fits between regular apps and gaming. Photo, music and video editing are the first that come to my mind. For some people, Macs are the preferred option due to Logic and Final Cut. When you don't have the $ for Mac Pro, iMac is your best option. In the end, iMac is far more powerful than MBPs or other laptops.

    Sure, AIO form factor has its downsides but it's pretty clear that Apple has never been interested on enthusiasts. For an Average Joe, iMac is a brilliant machine, which explains why it sells so well too.
  • joe_dude - Saturday, May 28, 2011 - link

    Don't be hoodwinked. Any 3-year old SLI rig would beat the pants off this thing. Remember, it's a _mobile_ CPU with a _mobile_ GPU; a laptop pretending to be a desktop.

    Any middle-of-the-road SFF/ITX machine with a Radeon 6850 would beat the pants off this thing. This thing is all about form over function. Save the money and get a 30" Dell monitor instead.
  • Kristian Vättö - Saturday, May 28, 2011 - link

    I would like to know where you can find a 3.4GHz quad core mobile CPU. The CPUs are standard desktop parts and i7-2600 is actually one of the fastest CPUs that is available today.

    Your 3-year-old SLI rig beat or come close in GPU performance but the iMac would run circles around it in CPU bound tests. If you talk about performance, then you must include every area, not just gaming and graphics tests. Or then specify that you are only talking about gaming performance.

    iMac's value isn't that bad when you consider the value of the screen. 30" monitor will cost you at least 1000$ so that leaves you another 1000$ for the actual computer. Sure, that is more than enough to get the similar components but the iMac won't end up being more than ~200$ more expensive (which is pretty good when talking about Apple).

    Nobody is forcing you to buy an iMac so that is why I don't get why you are complaining. Clearly, there are people who want one, even if it is a compromise. You can stick with your PC if that is what you prefer.
  • joe_dude - Saturday, May 28, 2011 - link

    Ah yes, you're right on the CPU. For some reason I thought the dual channel 1155 socket meant it was a mobile CPU.

    The Dell version of the 27" can be had ~$800. Of course, retail price is ~$1000, and Apple wouldn't charge less, of course.

    The 3.4 GHz i7 option costs extra (is it really $500???), so we're talking a ~$1700 PC (sans monitor). We're talking a full out i7 SLI rig vs. a weaksauce iMac for that price.

    I'm a gamer, but not crazy enough to spend that on an all-in-one!
  • Kristian Vättö - Saturday, May 28, 2011 - link

    I think you should really check your facts before you post. The i7 option in iMac is 200$, not 500$. Sure, it is still overpriced but BTOs in general are. The iMac can also be had for less when you look at resellers like MacMall so IMO, the only fair comparison is to look at retail prices.

    For 1200$, you can build a nice PC but definitely not an SLI system (unless you go with lower-end GPUs, though that kills the idea of SLI IMO).

    If we wanted to be fair, we should compare the iMac to an OEM PC, not self-built one. Why? Because you are paying for the labor and service in the iMac as well. That is why OEM PCs cost more than the ones you build on your own. I know the joy of building your own computer is unbeatable and I love it too but when comparing things, you have to understand where the costs come from. It's a whole new question whether it is worth it for you to pay the extra for service etc.

    I'm not trying to sell you an iMac but to make you understand where the costs come from and in the end, why the iMac isn't that bad value. For gamers, the iMac is and will always be a very compromised system.
  • donnyg - Saturday, May 28, 2011 - link

    As SmithJ mentioned, a i7 computer with PCIe 16x/4x is only around $750 USD and that's including a GPU.

    You can easily get 2x HD6950x/6970s which do much better in that resolution anyway because of the VRAM requirements.
  • KoolAidMan1 - Sunday, May 29, 2011 - link

    Throw in the same 2560x1440 IPS display and you're adding at least another $1000 to the whole package. Then there's operating system and the bundled software, and the fact that all-in-ones from any PC builder costs more than a standard ATX based PC.

    Given everything involved, the iMacs are actually a pretty good deal. Really fast, amazing display, tiny footprint, and silent. I still build my own PCs and I'll never give that up, but I love having it plugged into my iMac as a display and using both.
  • smithj - Saturday, May 28, 2011 - link

    My shop runs iMacs because they're just so good with size and noise - we're really damned pleased that we've ditched the old noisy workstations and they're fast enough for what we want to do!

    But he's kind of right. For a consumer, iMacs aren't the best of value. A small mATX computer with an i7 2600 (what 99% of people going to do with an i7 I don't know) and an AMD HD 5850 only costs around $700, and this isn't even looking around. Throw in a 27" Ultrasharp or ACD and your whole computer is going to cost only $1500-1700.

    They're popular because they're:
    - Small
    - Amazing looking
    - Perform well enough
    - Quiet
    - Covered by the best consumer warranty in the market
    - Good out of the box, no extra work required

    Its got nothing to go with specs, most people who buy them frankly don't care about the specs. $500-700 extra to pay for a generally weaker computer is a bit annoying but I'm getting old and frankly the warranty service, ease of use, and lack of noise is loved by all.
  • smithj - Saturday, May 28, 2011 - link

    We can't edit post. The point I'm making is that in a price/spec game, the iMacs are nowhere near as good as DIY PCs but there are some things that can't be directly put in a small HTML table.

    Apple to this day seems to be one of the few companies, if not the only, who understands this important factor.

Log in

Don't have an account? Sign up now