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

  • Mentawl - Friday, May 27, 2011 - link

    Hrm, I wouldn't call that backwards at all. The monitor is perhaps the single most important thing when interfacing with a computer, and it's worth splashing out on it over 10% extra CPU or GPU power or whatnot.
  • mcnabney - Friday, May 27, 2011 - link

    You missed the point. Definitely pay good money for a nice screen.

    However, in three years this nice screen on the iMac is going to be stuck on an outdated system. If you bought the system separate from the monitor you could save a huge cost (of having to buy ANOTHER expensive IPS screen) when upgrading to a new system.

    Is is actually kind of sad, knowing that all of these awesome screens are going to land in the junk heap in five years when they could provide excellent service for 10-20 years.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    The plus side is that iMac resale value is very high, and they update the displays every 2-3 years or so. Sell the old iMac on ebay for a good amount, and use the proceed to replace it with a new one with a better LCD.

    I upgrade my gaming PCs every 2-3 years, and I wish that upgrading it was as simple as with my iMac. With the iMac I put the whole thing in the box it came in, and the new one is faster with a better monitor. With my PC I have to sell components piecemeal for way less return than I get with my Mac stuff.
  • rubaiyat - Wednesday, September 7, 2011 - link

    So what do you do with your old PCs throw them away when you recycle the peripherals, and selected components? Not that there is much point to most of those.

    Macs go to a new home and the money from that pays a large part of a newer Mac.

    PC upgrade = 1 half new PC, plus box of discarded parts.

    Mac upgrade = 2 computers, 1 totally new, 1 older but still working.

    So which makes more sense? Which is more environmentally sensible?
  • kevith - Friday, May 27, 2011 - link

    I think it´s a bit like buying a stereo: Use half your cash on the speakers, and the other jalf on amp, CD-player and good cables.

    Then you have a well-matched system.

    And here it makes sense - to me at least - to spend one half on the screen.

    You´re gonna look at it several hours every day, and it´l probably outlive two or three builds ahead.
  • Spivonious - Friday, May 27, 2011 - link

    I think you should spend 80% on speakers, 20% on other stuff (and no more than $10 on cables. Really, they don't make a difference). Most amps today are equally good, and jumping from 100W per channel to 150W per channel is pointless when a normal music source will use 1-2W per channel. Even really blasting it will only use 5-10W. I'd much rather get better sound, and that comes with better speakers.
  • mcnabney - Friday, May 27, 2011 - link


    My speakers ~2k
    Receiver ~400
    BluRay ~120
    Cables ~50 for everything (12ga for speakers, 1 nice RCA for sub, the rest is cheap digital)
  • Exodite - Friday, May 27, 2011 - link

    Regarding the display resolution it's all in what you do.

    I mostly work with text and horizontal space is pretty meaningless for me, which means the only upgrade for my two 1280x1024 displays is to go for a 2560x1440 or 2560x1600 panel.

    There's no way in hell I'm paying the asking price for those though, I can get no less than *eight* 1080p displays for the price of one 27" 2560x1440 display. Mostly, I suspect, due to these kind of displays being aimed at graphics professionals and coming with all kinds of features that I care nothing about.

    I can only agree with Anand and hope that the strong focus on high-DPI mobile displays will trickle upwards too. After all, with 4" panels doing 720P and 10" displays doing quadruple that a 23-27" high-resolution display shouldn't be a problem.. right?
  • Rinadien - Friday, May 27, 2011 - link

    Or... you could get a 1900x1200 display, and rotate it 90 degrees?
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Those eight 1080p displays are TN. Sorry, not interested in downgrading, I'd rather have one high quality display instead of eight crappy ones. I have two IPS displays on my desk and I wouldn't trade them for any number of TN monitors.

Log in

Don't have an account? Sign up now