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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  • iwod - Friday, May 27, 2011 - link

    My problem is with Apple's Memory upgrade. They have 2x 2GB fitted which means if you want 8GB you either have to pay sily amount to apple, or buy 4GB x2 yourself and sell your original to someone else.

    As soon as the hardware encoder inside Intel can be accessed through Mac OSX, i believe 4 Core 8 thread will be enough for 99.9% of my task. Next year iMac GPU upgrade should be much more important then CPU IvyBridge.

    And only if Apple actually make Z68 Intel SRT to work. We need SSD, in cache or main drive.
  • setzer - Friday, May 27, 2011 - link

    Actually these iMacs have 4 so-dimm slots, so you should be able to get 8gb's fairly easily and cheap, though if I was changing the memory I would also change the hdd in one go, opening that can is hard work..
  • FATCamaro - Friday, May 27, 2011 - link

    Yeah I bought the 27" with the SSD upgrade and bought memory separately. SSD isn't that much more than retail and the imac is a pain to open. The memory is a bit of a ripoff though.
  • Zandros - Friday, May 27, 2011 - link

    Isn't the memory accessible through removing a single screw at the bottom of the iMac, though? No need to go in through the display just for that.
  • archer75 - Friday, May 27, 2011 - link

    Actually it's 3 screws ;)

    But there are 4 ram slots. I just bought 16gb from newegg and plugged it in. Cost less than what apple would charge for 8gb.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Anand - First off, great review as always.

    The conclusions you reached regarding the iMac's performance "finally" getting to an acceptable point is something that I came to with the release of the mid-2007 24" iMacs. Prior to that I had a PowerMac workstation for Final Cut Studio. The Core 2 Duo used in the 2007 iMac convinced me that an all-in-one would finally be a suitable replacement instead of a Mac Pro, and it was. It cost me much less than a Mac Pro while still giving me good performance and it had a 24" IPS display built in, which at the time ran $1000 by itself from NEC or Apple.

    The 27" iMac released in late 2009 was a similar deal: a powerful i7-860 machine combined with a $1000 27" IPS display. iMacs make upgrading simple too, you just pack the whole thing in the original box and ship it off. The only thing you have to mental your way past is letting go of the display, and that isn't hard for me based on the fact that the 27" display in the current iMac is spectacular. It was a more than worthwhile upgrade from my prior 24" iMac, and it didn't cost me too much given that I sold the old one for about 60% of what I paid.

    The 27" iMac display also functions as the primary display for my gaming PC. Unlike the iMac, my PC has SLI video cards so it can tear through that 2560x1440 res with no problem. Unlike the iMac, my PC isn't for work, it is for play. :)

    This brings about my main issue with the 2011 iMacs, and it isn't something that was addressed in this review. I'm talking about the new requirements for Target Display Mode (using the iMac as a monitor for an external source) now that the mini-DP port was replaced with a Thunderbolt port. The only sources that you can use on the new iMacs are those equipped with Thunderbolt ports. For the time being this limits you to 2011 Macs, that's it. No old Macs, and no PCs until next year at the soonest.

    Coming from someone that uses an iMac as an external display every day, that is pretty disappointing. Perhaps an active adapter or something will come out, but for the time being it really limits that aspect of the new iMacs.

    Thanks again!
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Sidetrack - I suggest testing Starcraft 2 with ultra textures and medium settings. My PC has plenty of juice to run with everything cranked, but unless you take a still frame and examine it, the difference in visual quality between medium and ultra settings is not noticeable. The difference in framerate is VERY noticeable though, which is why I run at medium settings with ultra textures. Nice and fast while still looking pretty.

    I'm very curious to see how the new iMacs would run with those settings. In my own experience you're looking at nearly a doubling in framerate, but that is also with an SLI setup that may not have the same performance delta as the single GPU in the iMac. I also saw a huge difference with my laptop, but I didn't examine the framerate as closely with it so I can't say for sure if it is "nearly double" in that case.
  • jonwd7 - Friday, May 27, 2011 - link

    The price comparison chart on the first page is wrong. I haven't made it through your review far enough to tell, but either you did not receive the high-end iMac, and you're quoting the wrong price ($1999) for the stats you've listed, or you did receive the high-end iMac, the price is right, but the stats are wrong.

    The stats you have listed are for the $1699 model.
  • krazyderek - Friday, May 27, 2011 - link

    i noticed this as well, specifically this means,

    the $1999 model currently features the 6970 1gb, and 3.1ghz quad core i5

    the $1699 model currently features the 6770 512mb not the 5xxx

    in fairness the $1499 model should really be used along with the $1199 to compare to those other price points.

    The $1999 model is in a completely different class and should be compared to it's appropriate competitors
  • psonice - Friday, May 27, 2011 - link

    I'd say this isn't really much of an issue. When most people upgrade, they sell their old box - and the mac holds its value incredibly well. So what actually happens is in a couple of years you sell the old box for a large chunk of cash, and spend a smaller-than-expected sum on the new one.

    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. This is assuming it doesn't break down - if that happens, they're very expensive ;)

    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.

    Oh, and I did upgrade mine to SSD (I've ignored that when looking at the prices). Yeah, it's a "fun" upgrade, and I ditched the HDD completely so I missed out on the 'removing the motherboard' stuff. Suction cups aren't actually required - you can get by with a pair of car windscreen mounts for GPS.

    An easier way to access the HDD would be very welcome - especially as the new ones come with a special SATA connector so you'd need to buy an apple-specific drive too! (Not sure if this was covered in the article, I skipped a lot of it, but it's a pretty major downside for us technical types - the average punter probably wouldn't care less).

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