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
joe_dude - Saturday, May 28, 2011 - link
But the thing's essentially a 3-year old PC. Why the heck would I want to pay more to downgrade?Of course Macs hold their resell value... it's not for gaming, so even a 5 year old Mac seem pretty fast using regular apps.
Even if it can be used as just a monitor, who the heck wants the extra dead weight? All-in-ones will always be a compromise. The point of having a desktop PC is *not* to compromise. Otherwise, a laptop can do the job.
Kristian Vättö - Saturday, May 28, 2011 - link
Good luck finding a Sandy Bridge CPU and AMD 6000-series graphics in a 3-year-old PC.A LOT fits between regular apps and gaming. Photo, music and video editing are the first that come to my mind. For some people, Macs are the preferred option due to Logic and Final Cut. When you don't have the $ for Mac Pro, iMac is your best option. In the end, iMac is far more powerful than MBPs or other laptops.
Sure, AIO form factor has its downsides but it's pretty clear that Apple has never been interested on enthusiasts. For an Average Joe, iMac is a brilliant machine, which explains why it sells so well too.
joe_dude - Saturday, May 28, 2011 - link
Don't be hoodwinked. Any 3-year old SLI rig would beat the pants off this thing. Remember, it's a _mobile_ CPU with a _mobile_ GPU; a laptop pretending to be a desktop.Any middle-of-the-road SFF/ITX machine with a Radeon 6850 would beat the pants off this thing. This thing is all about form over function. Save the money and get a 30" Dell monitor instead.
Kristian Vättö - Saturday, May 28, 2011 - link
I would like to know where you can find a 3.4GHz quad core mobile CPU. The CPUs are standard desktop parts and i7-2600 is actually one of the fastest CPUs that is available today.Your 3-year-old SLI rig beat or come close in GPU performance but the iMac would run circles around it in CPU bound tests. If you talk about performance, then you must include every area, not just gaming and graphics tests. Or then specify that you are only talking about gaming performance.
iMac's value isn't that bad when you consider the value of the screen. 30" monitor will cost you at least 1000$ so that leaves you another 1000$ for the actual computer. Sure, that is more than enough to get the similar components but the iMac won't end up being more than ~200$ more expensive (which is pretty good when talking about Apple).
Nobody is forcing you to buy an iMac so that is why I don't get why you are complaining. Clearly, there are people who want one, even if it is a compromise. You can stick with your PC if that is what you prefer.
joe_dude - Saturday, May 28, 2011 - link
Ah yes, you're right on the CPU. For some reason I thought the dual channel 1155 socket meant it was a mobile CPU.The Dell version of the 27" can be had ~$800. Of course, retail price is ~$1000, and Apple wouldn't charge less, of course.
The 3.4 GHz i7 option costs extra (is it really $500???), so we're talking a ~$1700 PC (sans monitor). We're talking a full out i7 SLI rig vs. a weaksauce iMac for that price.
I'm a gamer, but not crazy enough to spend that on an all-in-one!
Kristian Vättö - Saturday, May 28, 2011 - link
I think you should really check your facts before you post. The i7 option in iMac is 200$, not 500$. Sure, it is still overpriced but BTOs in general are. The iMac can also be had for less when you look at resellers like MacMall so IMO, the only fair comparison is to look at retail prices.For 1200$, you can build a nice PC but definitely not an SLI system (unless you go with lower-end GPUs, though that kills the idea of SLI IMO).
If we wanted to be fair, we should compare the iMac to an OEM PC, not self-built one. Why? Because you are paying for the labor and service in the iMac as well. That is why OEM PCs cost more than the ones you build on your own. I know the joy of building your own computer is unbeatable and I love it too but when comparing things, you have to understand where the costs come from. It's a whole new question whether it is worth it for you to pay the extra for service etc.
I'm not trying to sell you an iMac but to make you understand where the costs come from and in the end, why the iMac isn't that bad value. For gamers, the iMac is and will always be a very compromised system.
donnyg - Saturday, May 28, 2011 - link
As SmithJ mentioned, a i7 computer with PCIe 16x/4x is only around $750 USD and that's including a GPU.You can easily get 2x HD6950x/6970s which do much better in that resolution anyway because of the VRAM requirements.
KoolAidMan1 - Sunday, May 29, 2011 - link
Throw in the same 2560x1440 IPS display and you're adding at least another $1000 to the whole package. Then there's operating system and the bundled software, and the fact that all-in-ones from any PC builder costs more than a standard ATX based PC.Given everything involved, the iMacs are actually a pretty good deal. Really fast, amazing display, tiny footprint, and silent. I still build my own PCs and I'll never give that up, but I love having it plugged into my iMac as a display and using both.
smithj - Saturday, May 28, 2011 - link
My shop runs iMacs because they're just so good with size and noise - we're really damned pleased that we've ditched the old noisy workstations and they're fast enough for what we want to do!But he's kind of right. For a consumer, iMacs aren't the best of value. A small mATX computer with an i7 2600 (what 99% of people going to do with an i7 I don't know) and an AMD HD 5850 only costs around $700, and this isn't even looking around. Throw in a 27" Ultrasharp or ACD and your whole computer is going to cost only $1500-1700.
They're popular because they're:
- Small
- Amazing looking
- Perform well enough
- Quiet
- Covered by the best consumer warranty in the market
- Good out of the box, no extra work required
Its got nothing to go with specs, most people who buy them frankly don't care about the specs. $500-700 extra to pay for a generally weaker computer is a bit annoying but I'm getting old and frankly the warranty service, ease of use, and lack of noise is loved by all.
smithj - Saturday, May 28, 2011 - link
We can't edit post. The point I'm making is that in a price/spec game, the iMacs are nowhere near as good as DIY PCs but there are some things that can't be directly put in a small HTML table.Apple to this day seems to be one of the few companies, if not the only, who understands this important factor.