The 27-inch Apple iMac Review (2011)
by Anand Lal Shimpi on May 27, 2011 2:30 AM ESTIntel's Z68 Chipset, Thunderbolt & Display IO
Sandy Bridge integrates a GPU occupying roughly 20% of the quad-core CPU die. While what Intel calls its processor graphics is used on the MacBook Pro, outside of Quick Sync it's never used on the iMac. In fact, the processor graphics isn't even connected to the display at all.
Intel quad-core Sandy Bridge die
On the MacBook Pro Apple connects both GPUs to the display via a multiplexer and can instantaneously switch between which one is driving the panel without any frame buffer copying. With both GPUs connected and real time switching, accessing Quick Sync isn't an issue. On the iMac however, with only the discrete GPU connected, it is a problem. Apple claims this is why it used Intel's Z68 chipset: to access Quick Sync without a display output connected to the processor graphics. The Z68 based iMac actually went on sale before Intel's NDA on the Z68 chipset lifted. On the PC side we have Lucid's Virtu but for some reason Apple needs to use Z68 to achieve a similar end result. All other Z68 specific features go unused - there's no support for SSD caching and obviously no support for overclocking.
The other major change from the MacBook Pro is that the Thunderbolt controller no longer branches off of Sandy Bridge's on-die PCIe controller. Let's look at Intel's Z68 block diagram:
With Westmere (Clarkdale/Arrandale) Intel integrated a 16-lane PCIe 2.0 controller onto the processor die. The same controller made its way into Sandy Bridge. Traditionally these lanes are used for discrete graphics, the other major bandwidth hog within your system however the lanes are open to be used by PCIe device. In the 2011 MacBook Pro Apple used four of these lanes for Intel's Thunderbolt controller, leaving the discrete AMD GPU with a x8 interface. Even high end desktop GPUs aren't limited by a x8 PCIe 2.0 interface so there was no real performance penalty.
On the 2011 iMac however, Apple gives the discrete AMD GPU all sixteen lanes from the CPU. The Thunderbolt IC (which carries a different model number) branches off the Z68 chipset, which has 8 PCIe 2.0 lanes by itself.
Only four lanes are used by Intel's Thunderbolt controller, the remaining lanes are used for things like Bluetooth and WiFi. Do the math and you'll realize that four PCIe 2.0 lanes are only good for 20Gbps of bandwidth, plus DMI between the Z68 chipset and Sandy Bridge is limited to 20Gbps itself. A single Thunderbolt port is capable of 20Gbps of bandwidth (10Gbps in each direction), so that works out well (if you don't use any of the other PCIe devices in the system at the same time). While the 21.5-inch iMac has a single Thunderbolt port, the 27-inch model has two. That's a total of up to 40Gbps of bandwidth to Thunderbolt devices, but only 20Gbps to the controller itself. Don't be fooled by the presence of two Thunderbolt ports on the 27-inch iMac, you don't get any more bandwidth than you would on the 21.5-inch model - you can just hook up more displays.
Each Thunderbolt port on the iMac is good for at least one display output depending on what GPU you have driving it. The high end 27-inch iMac has two Thunderbolt ports, each of which is capable of driving two displays. Yes, you could theoretically have a 5-display setup driven off of a single iMac (given that even the upgraded iMac only has a 1GB frame buffer I wouldn't recommend doing such a thing).
The Thunderbolt ports aren't exclusively for video output, you can use one of the ports for video input. While you can use DisplayPort cables for video output, to make video input work you need to output video from a Thunderbolt port using a Thunderbolt cable. That means, at least today, the iMac can only accept video from a 2011 MacBook Pro (or technically another iMac) using a cable that doesn't yet exist on the market. You can expect to see Thunderbolt cables appear in stores very soon though.
Video aside, you will also be able to use the Thunderbolt ports for upcoming Thunderbolt devices as well. Each port supports a daisy chain for up to six Thunderbolt devices, meaning you can hook up a total of 12 devices to a 27-inch iMac.
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dagamer34 - Friday, May 27, 2011 - link
It's a desktop CPU, but a laptop GPU, and it really shows on page 4 when comparing the 6970M against desktop card. When you've spent $2000 on a machine with the same graphical performance as a $160 video card, that's when you REALLY know that Macs are NOT meant for gaming.That's why instead of buying an 27" iMac for gaming, I bought a 27" ACD and built a gaming PC. Same price, but PC parts (especially GPU) are upgradable, and since the computer isn't attached to the monitor, it retains it's value a LOT more.
Penti - Friday, May 27, 2011 - link
I would prefer the Dell U2711 here, as it's about 450 dollars less here in Sweden with Apples fucked up pricing tied to old exchange rate. Neither is it too fun with a 1650 dollar screen with just mini-displayport input. (Of which 25% VAT is included).Kristian Vättö - Friday, May 27, 2011 - link
Not a big deal but I thought I would add them anyway. On first page, you say 15" MBP has AMD 6770M while it really has 6750M. Scroll down to the HD part of the first page and it says the base 21.5" comes with 512GB HD, while it is a 500GB.For anyone who says I'm nitpicking, I'm not. I have written articles myself and I have made typos too. Look at the Ivy Bridge/Panther Point article's comments if you don't believe (I typoed that IB IGP will have OpenCL 10.1 :D). Like I said, it isn't a big deal but personally, I appreciate if someone points out my typos in a friendly matter.
BTW, Anand, you look like the mafia boss of SSDs in the FaceTime pic :D SSDs for every finger.
tipoo - Friday, May 27, 2011 - link
Also on the GPU page, "The entry level 21.5-inch MacBook Pro "...Now that would be an interesting product, lol.awaken688 - Friday, May 27, 2011 - link
"I've always kept displays through several upgrades, but you can't really do that with an iMac. I'm not really sure how to come to terms with that aspect of what Apple is offering here."That is the key statement. You have a nice $1000 monitor, but you have to sell it to upgrade (yeah, you can use it again, but you have a whole computer in the back). So you take depreciation on your hardware and depreciation on your monitor. Then you get another iMac and repeat. In the PC world of desktops, you get a nice monitor and only take depreciation one time on the monitor. Over 3 upgrade cycles, that can be $500-$1000 in savings over the iMac solution depending on the quality of the monitors. That is a big deal. MacBook Pros make perfect sense to me, but Apple just does not offer a desktop model that fits my needs. Mini is too slow and Mac Pro has Xeon cores which I refuse to pay for as I don't need them. I won't hold my breath for Apple to fill in my needs.
Kristian Vättö - Friday, May 27, 2011 - link
Mac Pros just have an extremely fat profit margin, nothing else. Xeon 3000-series CPUs cost as much as their Core iX counterparts. For example the W3530 used in base Mac Pro costs 294$, which is the same as what i7-930 costs. Dell sells a similarly equipped workstation for around 1500$, and yes, that includes Xeons, ECC RAM, workstation GPU (something that MP doesn't have) etc.It's obvious that iMac is Apple's flagship in consumer desktop market. They have shown zero interest towards a mid-tower though why would they? iMac is selling brilliantly.
dagamer34 - Friday, May 27, 2011 - link
At this point, Mac Pros are pretty much in dire need for a price adjustment. Even if you really do need all that power, I think buying it makes you feel silly compared to what is available in the MacBook Pros and iMacs of today. And with Thunderbolt, the biggest reason to buy a Mac Pro has disappeared (high speed i/o cards).Penti - Friday, May 27, 2011 - link
Well when they still where new, the dual processor models where priced competitively against real HP and Dell workstations which often even did cost a bit more. The problem here are two things, the single processor model is just rubbish and priced about 1000 dollars too much (a year ago or even two years ago) and that Apple never adjusts the price of a model but instead replaced them with a new one. With a new price.A single socket Mac pro shouldn't be more then a C i7 2600K for like 1200 dollars now, a extreme edition SNB would cost some additional 700 dollars, dual processor model should use something like Westmere-EX by now. 10-core (6-10C) two socket support and quad-channel memory. Why mess around with LGA2011 or LGA1366 today? They pretty much have no choice but to go real high-end or use normal desktop parts with the Mac Pro update. There are no Sandy-Bridge workstation class processors. And dual 8C Westmere-EX would end up costing something like 6000 dollars for the machine though. There's just no good workstation hardware competitively priced there to begin with right now. Right now it doesn't get better then dual Westmere 2.93GHz as the Mac Pro uses. AMD HD6970, two 8-core Westmere-EX is pretty much as far they could go today/this year and that would end up costing at least above 5000 dollars. Just leaving iMac comfortably under that as workstation. But they probably won't upgrade that until sometime after Lion any way.
KoolAidMan1 - Friday, May 27, 2011 - link
The thing is that depreciation on the iMac is much much slower than it is with PC parts. I sell my iMac and my gaming PC components at roughly the same time, and what I make back selling the old iMac is significantly higher than what I sell my PC parts for.Getting a high resale return on my old 24" iMac and using the proceeds to get a new 27" iMac with that gorgeous display was a great deal, and it actually sold me (I was skeptical too) on the idea of upgrading all-in-ones by selling the whole thing on ebay. Getting a similar return on my PC is just not possible.
tipoo - Friday, May 27, 2011 - link
Does anyone know if the GPU switching is enabled in the iMac's? They didn't mention the HD graphics on the spec page unlike the MBP's, so maybe they didn't bother with it since there is no battery. Also most of them use the HD2000 which is half as powerful as the HD3000, so maybe it didn't meet their requirements even for basic desktop work.