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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  • smithj - Sunday, May 29, 2011 - link

    People not within the European Union do not pay the VAT tax. If you live outside of the European Union, you pay £369.00 which is around $570 USD/AUD so he's completely correct.

    Even if you live in the European Union, you're going to be saving a similar amount since I imagine Apple products get the VAT tax slapped onto them as well. Doesn't change anything.

    On the Hazro website:
    "If you are ordering from outside the UK and EU, you will not be charged VAT."

    There are extremely good reasons for getting a iMac but from a parts point of view its terrible value. You can't measure the benefits of the iMac however with just plain figures and specs.
  • Kristian Vättö - Sunday, May 29, 2011 - link

    Ahh, that explains it then. I didn't look any further so didn't know about the international price. Customs may be added to the price though (not sure about the US policy as you don't VAT. If I buy something from the US, I will have to pay the Finnish 23% VAT before I can receive it).

    The Hazro looks very good though, thanks for linking it. Something to keep in mind when this oldish iMac needs to go. 500€ for a 27" 1440p display doesn't sound that bad, could get two of them for the price of one 27" ACD
  • repoman27 - Sunday, May 29, 2011 - link

    I took issue with OWC's blog posts about the HDD upgrade issues in the 2011 iMac, and was hoping to find a better clarification here than the one in this review. OWC insisted on using the terms "standard", "leads" and "pins" in odd ways, and some of that seems to be echoed here.

    The HDD power cable in the iMac (Mid 2011) has 7 conductors, a standard 15-pin SATA power connector on one end, and a small 7-socket connector on the other which attaches to a header on the logic board. This assembly, while proprietary, is not necessarily any more so than the ones that ship with modular PSUs. It is more common to see SATA power cables with only 4 conductors (GND, 5 V, GND, 12 V) or 5 conductors if 3.3 V is also supplied. Several pins are then ganged together in the 15-pin connector to support the specified current requirements. As noted in the article, Apple is using relatively small conductors, and thus 2 are used in tandem for 12 V, they also elected to use the reserved pin 11 of the 15-pin SATA power connector for temperature monitoring. Pins 10 and 12 must both be connected to ground, although most of the time one conductor is used and pins 10-12 are all ganged together. Apple couldn't do this, so instead their 7 conductors are for GND, 5 V, GND, Temp, GND, 12 V, 12 V.

    It is not common behavior for HDDs to output a temperature signal on pin 11 of the SATA power connector. While the model numbers and controller boards of the drives that ship in the 2011 iMacs look to be no different from retail ones, there would appear to be differences in the firmware that allow for this functionality. I would imagine that patching the firmware of a retail drive is possible.

    If you ground pin 11 of the 15-pin SATA power connector, SMC will act as if there is no 3.5" drive installed. This means that the speed of the HDD fan will not increase above a certain baseline, and if you really do have a 3.5" drive in that bay, it will not properly dissipate the heat generated by it. If you use this trick to swap out the stock drive for an after-market one, you should also use S.M.A.R.T. based fan control software.

    There are separate, clearly labeled power headers for HDD, ODD and SSD on the iMac's logic board. It is probably a better idea to use the dedicated SSD power header than a Y-splitter if you add a 2.5" drive to an iMac that didn't ship with one installed. The HDD cable was more than likely not designed to support two devices, and most Y-splitters for SATA power cables gang pins 10-12, thus grounding pin 11 and preventing the SMC from receiving HDD temperature data. The SSD power header on the 27-inch models is a standard 6-pin slimline SATA power connector, but on the 21-inch it is just a small 4-pin header.

    The design of the iMac in general deters the casual from servicing their own hardware, and while some are discouraged by this, it's a ridiculous stance to take. Most people who purchase a new Mercedes S-Class don't worry about the fact that they probably won't be able to service it themselves (even if they are a fairly competent mechanic), or that it contains proprietary cable assemblies. If you don't feel comfortable servicing or modding your gear yourself, just take it to someone who does.
  • MadMinstrel - Sunday, May 29, 2011 - link

    A 512MB <i>framebuffer</i>? Really? So that's all you think GPU memory is useful for? That might have been the case in the good old S3 ViRGE days, but right now it <i>really</i> doesn't work that way and I would expect someone writing for AnandTech to know that. In fact, GPU memory holds at least two framebuffers and sometimes as many as 6. More if you count all the intermediate rendering steps such as depth buffers, accumulation buffers, G-buffers, albedo buffers, etc, as used in modern deferred rendering engines. Textures, geometry, physics data, and sometimes even some of the game logic also go into GPU memory. Maybe I'm just being pedantic about this but I guess it really annoyed me because I actually <i>care</i> about Anand being accurate.
  • Sunburn74 - Monday, May 30, 2011 - link

    Not for me thanks. Too pricey and lousy specs. I also don't care about TN vs IPS panels. Most people don't care. Some enthusiasts do, but you can't really argue that iMacs are targeted for the average joe who don't know a ton about pcs and just wants it to work whilst arguing that the average joe can appreciate a 500 dollar difference between an IPS panel and TN panel. The truth is most people don't even know there are 2 types of panels floating about.

    I consider myself an enthusiast. I've spent a couple months working on a last gen 27inc iMac in the past as a reference point, with a lot of adobe studio stuff. Long story short, I wasn't impressed. I found the machine to be slow to average in terms of performance (compared to the HP tower workstations we had down the hall for "real work" haha). Throw out the nicer display (who's value is questionable depending on who you talk to. Again I've read in the comments how imacs are great for your dad and mom and your granny.Your granny is not noticing a difference between a 27inch HP tn panel and a 27inch mac IPS panel) and I don't really see what the iMac brings to the table that all the other all in ones don't have. I just don't really see a good target audience outside of mac enthusiasts who just gotta have it.

    Its not great for people who need a pc that just works without hassle but are limited by budget (your mom, your dad, your granny, your girlfriend in college)

    Its not great for the power users like myself who render and process in different ways thousands of high def images a day for scientific purposes. The HP workstation crushed the mac with its superior specs and apparently less aesthetic screen (a high end HP 30 inch TN panel).

    Its not great for the true computer enthusiasts who likes to know whats in his pc and enjoys servicing his own box.

    the iMac is truly like the mercedes benz and not in a good way. Mercedes make nice cars, but no one can disagree that those cars (just like the iMac) are
    1)For the rich for whom cost is not truly an issue
    2)for those who don't have an interest in servicing and maintaining their own cars and will have the dealer do it all,
    3) For those who don't use their cars for actual work where the performance of the car actually matters (otherwise they'd go with the mac workstation or any othre workstation or build their own).

    The imac just puzzles me and yet it sells so well apparently.
  • JFA - Tuesday, July 26, 2011 - link

    It would be great to see comparison with previous generation of iMacs. Nowadays one can find quite a discounts on previous gen.
  • javaporter - Tuesday, September 27, 2011 - link

    I didn't read all 14 pages of comments, someone probably mentioned this, but you might want to look at dropbox. I have the 50GB option. It's enough for core project folders, my 1Password keychain, and various things I want to always have access to.

    I had the same issue as the author when deciding to leave a Mac Pro for a Macbook Pro. I did and worked that way for a while, however, recently I went back from a MBPro to an iMac and a Macbook Air.

    With dropbox, I don't really have to worry about moving files anymore. It's still not perfect; I still have to worry about 2 machines to update software on and my photos won't fit on my MBAir of course (nor will iTunes), but for me, it's still worth it to have a desktop. I liked my MBPro, but it just never seemed to be as fast or easy as the desktop or as portable as the Air (I had the 17" — pretty unusable when flying coach).
  • timmillea - Monday, December 19, 2016 - link

    So, five years later and this is still a very attractive machine. It is the last in the line of iMacs that do not need a 'pizza cutter' and replacing double-sided tape to obtain access to upgrade. Also the last with an optical drive which is still handy for ripping movies, music and sharing the same with people who are not riding the Apple wave. A 3.4 GHz quad-core i7 with a (ideally) 2GB GPU is still a very serious amount of computing power. Upgrade to 2 x SSDs with guides freely available on the net and configure them as Raid 0 and you have Mac-Pro challanging performance with a superb display in the size of the display alone available on the Bay for a third of the original price.
  • Rendell001 - Sunday, July 29, 2018 - link

    Thats very reassuring to know timmillea, I have the same model too which I picked up on eBay back in 2014 for £900. I love it for the same reasons you do though I've not tinkered with the insides yet. Needed to know if it would still hold up even now in 2018.

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