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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  • stm1185 - Friday, May 27, 2011 - link

    So basically to get rid of having a tower, which probably can take up floor space you don't use anyway, you get about $700 worth of desktop hardware and a $1000 monitor rolled into one at the price of $2000. Which does not seem that bad of a deal, except I could never see myself having a monitor that costs more then the computer used with it. It seems very backwards.

    For instance is the experience with having 2560x1440 resolution over 1920x1080 better then the experience that you get with $1600 worth of hardware over $700? I think I would say No.
    Reply
  • MrBigglesw0rth - Friday, May 27, 2011 - link

    Did you stop to consider what youre going to get for that 900$ more hardware cost? The main thing would be an SSD, then double the RAM. After that, what? Better speakers? Another optical drive? Expensive headsets? Maybe $300+ for 15% better processing power? How about a new paintjob on your car?

    Dont be silly. The best investment would be something you look at constantly; the display. When youre looking at minimal computational gains over a vastly better viewing experience for 2-3 new computer builds to come.

    Also, this isnt 1995 anymore. You can get 80% of the power for 20% of the cost. The largest cost in a good system today is a quality SSD and a quality screen, followed by the CPU, mobo/ram, etc.
    Reply
  • aguilpa1 - Friday, May 27, 2011 - link

    Fine a display is important and that is why I have 3 of them at 5760x1080, 120Hz Alienware Optx23's but ONLY if the displays are independent of the system, meaning not built in whereby the computer becomes obsolete and then what??? How are you going to upgrade the motherboard on a proprietary and overpriced all in one? Proprietary junk will never be of great value no matter how pretty they are. It's been tried again and again.

    Keeping your old monitor that is still good to save money makes sense but not on an all in one unit. It never has and never will.
    Reply
  • harshbarj - Friday, May 27, 2011 - link

    That is a good point! I have been running the same lcd now for 7 years and this is the 5th desktop to use this monitor. It was the first high end gaming 19 inch lcd on the market when I got it and even today it still looks great. Had I gone the imac way (which I would never as I play games) and bought an all in one, I would have needed to buy a new monitor with each new system. Reply
  • headbox - Friday, May 27, 2011 - link

    whatever. You can get a 23" widescreen display for barely over $100 now. You're just not in the crowd of people that can afford upgrades. All of these price criticisms always come from people that Apple doesn't market towards anyways: broke nerds. For MANY people, $2000 is not a big deal for a computer every 2 years (or less.) Reply
  • harshbarj - Friday, May 27, 2011 - link

    Really? Best price on newegg is about $150 and that monitor is noticeably smaller in height than my 19 inch. Anyway why replace something that still works just fine? Seems kind of stupid to me!

    Also for MOST people $2000 is a big deal.
    Reply
  • samirotiv - Saturday, May 28, 2011 - link

    buddy, this is a 27 inch LED backlit IPS display. What you're talking about is a TN display. Reply
  • samirotiv - Saturday, May 28, 2011 - link

    Don't get me wrong, I do agree with you. 2000USD is an investment. Some people have very basic performance needs, and can live with the same machine for 5 years. They value a good display, way more than a fast CPU. They attribute almost no importance to a GPU. A Mac takes almost no space, has almost no cables, no clutter. Some people value that.

    Besides, macs have a decent resale value too. But if I wanted a machine for myself, I wouldn't take a mac. I'd go for a custom built one.
    Reply
  • utlragear - Monday, June 11, 2012 - link

    Well it's not like blowing $2000 for an imac will get you major performance anyway. Apple only ships mdgrade hardware and it's always been that way. They pocket an extra $1000 off each isucker, and that can be PROVEN and broken down in itemized price lists. If one must, just build a PC for $700 that runs rings around an imac. Then hackintosh it and it will be faster than the one apple ships for $2000 by far. PROOF that they do exactly what I'm saying they do. But I can't see any reason to run an OS that is no better than win 7. If you want someone that has almost no cables get an ALL IN ONE PC. They are not like that because they are Macs. Macs are simply PC's anyway. There is NO value in it simply being apple. That statement will make some people mad, but sometimes when you break up someone's fantasies they also get mad. Reply
  • KoolAidMan1 - Sunday, May 29, 2011 - link

    A 27" 2560x1440 IPS display with LED backlighting is going to cost a lot more than a 23" 1080p LCD with a TN panel.

    Shocking.
    Reply

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