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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  • meorah - Friday, May 27, 2011 - link

    "Example: My 2007 24" imac was ~£1200. Resale value for similar spec on ebay today: ~£650-700. That means it's cost me around £500-£550 over 4 years, roughly the cost of a low-end desktop with an OK screen."

    its cost to you was 1200. it has depreciated around 500-550 over 4 years.

    If you were trying to lease an imac for 500-550 over 4 years, then it would have cost you 500-550 over 4 years, but you bought it so that's not right.
  • psonice - Friday, May 27, 2011 - link

    What difference does it make? At some point (probably soon) I'll sell it and buy a new box. At that point it'll have cost me 500-550, for 4 years of use.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Depreciation on PC parts is much worse. I sell my old PC components on roughly the same schedule as my Macs, every 2-3 years, and with my Mac sales it is more than enough to help pay for its replacement. With my old PC parts, not so much. :)
  • KoolAidMan1 - Friday, May 27, 2011 - link

    You nailed it. You aren't "losing a good monitor" when you sell your old iMac for a new one, you're getting a better monitor with the major revisions.

    The 24" iMac had a great H-IPS panel in it, but the upgrade from that to the 27" IPS panels in the new iMacs is well worth the upgrade. Combine that with high resale value on Macs and its a pretty good deal, ridiculously easy upgrade too (just pack the old one in the box it came in).
  • DarkShift - Saturday, May 28, 2011 - link

    "Basically, macs are surprisingly cheap when you factor in the resale value. You either keep them long term (and they pay for themselves then anyway), or you sell after a few years and get half your money back."

    That's surpsrising considering that Mac's are mostly underpowered even as new. 650£ for 2007 iMac is way too much considering how slow it must be.

    I have noticed, that most people who happily buy macs really don't know anything about tech stuff. Many still think that there must be something in Apple hardware that's better than in PC's while they often share same components.

    For comparison, my self build PC workstation runs circles around these iMac's and it cost me less. And that is with Intel i7 2600K @ 4.6Ghz, 3 SSD drives,16GB ddr3 ram, Blue ray and USB 3.0 ports. And absolutely no blue screens after 5 months use. ;)

    Benchmarked results:
    Retouch artics Photoshop (with CS5): 9,5s
    Cinebench R10 Rendering single: 7690
    Cinebench R10 Rendering multi: 30536

    Performance is the most important thing for pro users at it tells how fast you get your job done. Other issues are mostly cosmetic as most pro software is found for both Mac and PC. You get paid for using the tools, not for using them on specific OS.
  • jonwd7 - Friday, May 27, 2011 - link

    Unless I am mistaken, your claim that the SSD in the 2011 iMac is the same old Toshiba one they've been using is pure speculation, but you don't treat it like so. If you attempt to order a new 2011 iMac with an SSD, the shipping date gets moved back significantly. There is some possibility that this is because they are switching to a newer, possibly Samsung-branded SSD. It being Samsung is just a rumor I believe, based on what they used in the newest MacBook Airs.
  • kevith - Friday, May 27, 2011 - link

    I think it´s not a drawback, but quite the opposite, when a laptop or an all-in-one is fitted with too little RAM and/or too little HDD/SSD, since it´s the only things you can upgrade yourself.

    And that always cheaper than the price-premium the manufacturer will charge, certainly if the manufacturer is Apple...

    So for my part I always look for laptops without SSD and with as little memory as possible.
  • tech6 - Friday, May 27, 2011 - link

    Looks like a nice system that is held back by a design problem. Most laptops have easy access service panels for RAM and disk upgrades or replacements and the lack of this feature would rule this system out for me. That's a pity as it looks good and is reasonable value but if you have to remove the LCD and board just to get at the disk, that is just plain stupid industrial design.
  • Johnmcl7 - Friday, May 27, 2011 - link

    It's worse than stupid, it's entirely intentional as you're not supposed to repair or upgrade your iMac you're supposed to buy a new one. I cannot stand Imacs for their appalling internal design and I'm surprised a tech site like this can still praise Imacs given that laptops a fraction of the size are a two second job to get the drive out so there's absolutely no reason for the Imac to be any different.

    Even putting the Imac's terrible design aside, I'm not a fan of all in one PCs as I struggle to see the point unless you're really, really tight on space. You're essentially getting the disadvantages of both a laptop and a desktop but none of the advantages as the system is neither portable nor flexible/upgradeable or offering topend performance. I have a Dell U2711 which thanks to having just about every input possible can currently hook up to a few different machines and I expect it to last far beyond the current desktop PC it's mainly hooked up to. This new Imac seems even worse for use beyond the builtin computer with a very limited video input.

  • wintermute000 - Friday, May 27, 2011 - link

    The main issue is that its still laptop-esque price/performance for a desktop.

    The lack of an apple tower or upgradable box is quite astounding. They could just keep it single socket, 8Gb RAM or under, and consumer (not pro variant) gfx cards.

    2k USD can buy you a liquid cooled quad-core sandy bridge, mid-high GPU, SSD rig + a decent 24" IPS display with a spare 4 or so Tb of spinning platter storage. No contest except for OSX tax if your apps demand OSX. Back in the XP days the OS was worth the markup but no more IMO

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