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.
Facebook
Twitter
RSS
139 Comments
View All Comments
tipoo - Friday, May 27, 2011 - link
Nevermind me, it was answered in the article.tipoo - Friday, May 27, 2011 - link
I heard that unlike previous iMacs, the new ones can only use another mac with Thunderbolt to use the iMac as a targeted display. Is that true? I wouldn't feel so bad about discarding such a system when the GPU and CPU feel too old in two or three years like Anand mentioned if I could use it as an external display, but I think the new ones are limited to only being used by other Macs. And that's also assuming Mac's in 3 years will use compatible Thunderbolt ports.TegiriNenashi - Friday, May 27, 2011 - link
What is wrong with this display? Just one number: 16:9.QuietOC - Friday, May 27, 2011 - link
I have used a 20" white iMac and 24" aluminum iMac. The later has a big persistent image problem. Evidently IPS pixels don't work well in a hot environment. The low noise level of the iMac is nice to mostly not hear, but the visual noise might be worse.Also the cheap 320GB WD Caviar failed by randomly disappearing which may have be heat related, and the USB ports have also started randomly disconnecting. The mouse and keyboard just stop working during the day and I have to unplug them and plug them back in. So, no, I would not recommend getting a $1000 monitor with a computer mounted inside it.
KoolAidMan1 - Friday, May 27, 2011 - link
I've owned both the 24" iMac you talk about and a 27" from late 2009. Image persistence became a problem with the 24" models, as well as the CCFL losing some of its brightness by the two year mark.The new 27" models do not have either issue. Image persistence has been fixed, and I don't expect there to be any fading since LED backlights don't suffer from the same degredation issues that CCFL backlit displays do over time. On a related note, I have a NEC 2490WUXi as my secondary display. That monitor uses the same 24" H-IPS panel that the old 24" iMac did. It has minor image retention issues, but not to the same degree as the iMac had. Whatever LG did with their new 27" panels seems to have addressed that problem.
zhill - Friday, May 27, 2011 - link
First off, nice article Anand. Well presented and I think your perspective is a common one in this case. I agree that the lack of upgrade options and rather mediocre gpu performance are certainly off-putting for a machine in this price range, but I also think that your observation that if you are willing to spend $1800 for a laptop with a reasonably short lifecycle then the iMac is not much different. I think you've really hit the point there--the iMac's target customer.I would argue that most iMac users are not highly technical, power users. They are people that want a big screen and don't need the portability of a laptop. These people, like my parents, value the simplicity and ease of use of the iMac and the fact that their workspace is often small and would rather it not be covered in cords and cables. In that case not only does the iMac make sense but it's lifespan is substantially longer and all the GPU they need is enough to drive the system and maybe do some video editing. Gaming prowess has never been an apple concern and I doubt it will be until Steve Jobs decides to buy Activision or EA. I do like the iMac from a compute appliance perspective, just plug it in and compute--no fuss. Also, trying to make a reasonable gaming rig with a 2560x1440 display is a fairly daunting task even with today's cards. You would have to be near the top-end and that's a whole lot of power and heat to dissipate in a reasonably small enclosure (considering the size of a 6970 or 580 card by itself).
I also have to say that Intel's recent willingness to keep switching chip sockets has made upgrades far more painful than they should be (yes, I have a core i7-920 with socket 1366 that is now essentially orphaned).
Alberts - Friday, May 27, 2011 - link
Securedoc for Mac from a company called Winmagic supports SSD's with encryption hardware as long they adhire to the opal specification from the trusted computing groupJimmiG - Friday, May 27, 2011 - link
You can't upgrade the hardware and you can't separate the computer from the display so you can keep using the display long after the hardware has become obsolete.That's pretty much the oposite of "green".
Spazweasel - Friday, May 27, 2011 - link
This may be old news (I'm not going to wade through 6 pages of Apple-hating trolls to see), but you CAN use your 27" iMac as a monitor:http://support.apple.com/kb/HT3924
It's called "Target Display Mode".
KoolAidMan1 - Friday, May 27, 2011 - link
Note that it is only talking about the 2009 and 2010 models with mini-DP ports. Those can take an external source so you can use the 27" iMac as a monitor. I use it every day as the primary monitor for my gaming PC.The new 2011 iMacs have different requirements since they switched to Thunderbolt ports. Until an adapter or something comes out, the only sources that can output a video signal to the 2011 iMacs are Thunderbolt equipped computers. For the time being this only limits them to 2011 Macbook Pros and other iMacs (which would be a weird application).
I love the iMacs, but as someone who uses Target Display Mode every day, the new requirements bother me. It won't be a problem in a year or so when Thunderbolt becomes more common, but for the time being it is pretty limiting.