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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  • dagamer34 - Friday, May 27, 2011 - link

    It's a desktop CPU, but a laptop GPU, and it really shows on page 4 when comparing the 6970M against desktop card. When you've spent $2000 on a machine with the same graphical performance as a $160 video card, that's when you REALLY know that Macs are NOT meant for gaming.

    That's why instead of buying an 27" iMac for gaming, I bought a 27" ACD and built a gaming PC. Same price, but PC parts (especially GPU) are upgradable, and since the computer isn't attached to the monitor, it retains it's value a LOT more.
  • Penti - Friday, May 27, 2011 - link

    I would prefer the Dell U2711 here, as it's about 450 dollars less here in Sweden with Apples fucked up pricing tied to old exchange rate. Neither is it too fun with a 1650 dollar screen with just mini-displayport input. (Of which 25% VAT is included).
  • Kristian Vättö - Friday, May 27, 2011 - link

    Not a big deal but I thought I would add them anyway. On first page, you say 15" MBP has AMD 6770M while it really has 6750M. Scroll down to the HD part of the first page and it says the base 21.5" comes with 512GB HD, while it is a 500GB.

    For anyone who says I'm nitpicking, I'm not. I have written articles myself and I have made typos too. Look at the Ivy Bridge/Panther Point article's comments if you don't believe (I typoed that IB IGP will have OpenCL 10.1 :D). Like I said, it isn't a big deal but personally, I appreciate if someone points out my typos in a friendly matter.

    BTW, Anand, you look like the mafia boss of SSDs in the FaceTime pic :D SSDs for every finger.
  • tipoo - Friday, May 27, 2011 - link

    Also on the GPU page, "The entry level 21.5-inch MacBook Pro "...Now that would be an interesting product, lol.
  • awaken688 - Friday, May 27, 2011 - link

    "I've always kept displays through several upgrades, but you can't really do that with an iMac. I'm not really sure how to come to terms with that aspect of what Apple is offering here."

    That is the key statement. You have a nice $1000 monitor, but you have to sell it to upgrade (yeah, you can use it again, but you have a whole computer in the back). So you take depreciation on your hardware and depreciation on your monitor. Then you get another iMac and repeat. In the PC world of desktops, you get a nice monitor and only take depreciation one time on the monitor. Over 3 upgrade cycles, that can be $500-$1000 in savings over the iMac solution depending on the quality of the monitors. That is a big deal. MacBook Pros make perfect sense to me, but Apple just does not offer a desktop model that fits my needs. Mini is too slow and Mac Pro has Xeon cores which I refuse to pay for as I don't need them. I won't hold my breath for Apple to fill in my needs.
  • Kristian Vättö - Friday, May 27, 2011 - link

    Mac Pros just have an extremely fat profit margin, nothing else. Xeon 3000-series CPUs cost as much as their Core iX counterparts. For example the W3530 used in base Mac Pro costs 294$, which is the same as what i7-930 costs. Dell sells a similarly equipped workstation for around 1500$, and yes, that includes Xeons, ECC RAM, workstation GPU (something that MP doesn't have) etc.

    It's obvious that iMac is Apple's flagship in consumer desktop market. They have shown zero interest towards a mid-tower though why would they? iMac is selling brilliantly.
  • dagamer34 - Friday, May 27, 2011 - link

    At this point, Mac Pros are pretty much in dire need for a price adjustment. Even if you really do need all that power, I think buying it makes you feel silly compared to what is available in the MacBook Pros and iMacs of today. And with Thunderbolt, the biggest reason to buy a Mac Pro has disappeared (high speed i/o cards).
  • Penti - Friday, May 27, 2011 - link

    Well when they still where new, the dual processor models where priced competitively against real HP and Dell workstations which often even did cost a bit more. The problem here are two things, the single processor model is just rubbish and priced about 1000 dollars too much (a year ago or even two years ago) and that Apple never adjusts the price of a model but instead replaced them with a new one. With a new price.

    A single socket Mac pro shouldn't be more then a C i7 2600K for like 1200 dollars now, a extreme edition SNB would cost some additional 700 dollars, dual processor model should use something like Westmere-EX by now. 10-core (6-10C) two socket support and quad-channel memory. Why mess around with LGA2011 or LGA1366 today? They pretty much have no choice but to go real high-end or use normal desktop parts with the Mac Pro update. There are no Sandy-Bridge workstation class processors. And dual 8C Westmere-EX would end up costing something like 6000 dollars for the machine though. There's just no good workstation hardware competitively priced there to begin with right now. Right now it doesn't get better then dual Westmere 2.93GHz as the Mac Pro uses. AMD HD6970, two 8-core Westmere-EX is pretty much as far they could go today/this year and that would end up costing at least above 5000 dollars. Just leaving iMac comfortably under that as workstation. But they probably won't upgrade that until sometime after Lion any way.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    The thing is that depreciation on the iMac is much much slower than it is with PC parts. I sell my iMac and my gaming PC components at roughly the same time, and what I make back selling the old iMac is significantly higher than what I sell my PC parts for.

    Getting a high resale return on my old 24" iMac and using the proceeds to get a new 27" iMac with that gorgeous display was a great deal, and it actually sold me (I was skeptical too) on the idea of upgrading all-in-ones by selling the whole thing on ebay. Getting a similar return on my PC is just not possible.
  • tipoo - Friday, May 27, 2011 - link

    Does anyone know if the GPU switching is enabled in the iMac's? They didn't mention the HD graphics on the spec page unlike the MBP's, so maybe they didn't bother with it since there is no battery. Also most of them use the HD2000 which is half as powerful as the HD3000, so maybe it didn't meet their requirements even for basic desktop work.

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