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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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).