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Intel's Core 2 Extreme QX6700 processor


Quad-core computing arrives on the desktop
— 12:00 AM on November 2, 2006

YOU'VE GOTTA LIKE Intel's Core 2 Duo processors. After struggling mightily with performance and power consumption problems in the latter-day Pentiums, Intel came roaring back with the Core 2 Duo, producing a chip that goes like stink without spinning the electric meter into a frenzy. Since it offers a better combination of processing power, energy efficiency, and overclocking headroom than the Athlon 64, the Core 2 Duo has quickly become an enthusiast favorite, capturing prominent spots in our system guide recommendations and prompting a new round of upgrades for many folks.

Now comes the CPU de grĂ¢ce, a processor that takes advantage of the Core 2 Duo's modest heat output by cramming two of those chips together into a single socket, a product Intel can plausibly claim is the world's first quad-core CPU. The Core 2 Extreme QX6700 isn't exactly cheap and doesn't run especially cool, but it will turn your spare bedroom into the computing equivalent of a government astrophysics lab and make the neighbors terribly jealous—provided your neighbors are total geeks.

What hath Intel wrought with this quad-core beast? Do four CPU cores make sense in a desktop PC, and what sort of applications can really take advantage of such power? Let's have a look.

Core 2 Duo times two equals Kentsfield
We won't dwell too long on the specifics of the Core 2 Extreme QX6700. This product, which lived its early life going by the code-name Kentsfield, really is two Core 2 Duo chips mounted together on the same package. If you want to know more about the Core 2 Duo's basic technology, I suggest you read our review of that processor. Intel has used this multi-chip packaging technique in the past to create "dual-core" processors, such as the "Presler" Pentium D. Lashing together two separate chips rather than making one large chip makes good sense from an economic standpoint, because smaller die areas tend to make for higher yields of good chips from each wafer.

The result of this multi-chip fusion is a processor that plugs into a regular LGA775-style socket and packs four processing cores alongside a total of 8MB of L2 cache. Cosmetically, it looks for all the world like any other recent Intel desktop CPU.


The Core 2 Extreme QX6700

But here's a fancy illustration Intel came up with to show what's under the hood:

The Core 2 Extreme QX6700 runs at 2.66GHz on a 1066MHz front-side bus, so its clock speed matches that of the second fastest Core 2 Duo, the E6700. (The Core 2 Extreme X6800 is the fastest at 2.93GHz.) Intel probably chose not to push any harder on clock speed in order to keep the QX6700 inside of a reasonable power envelope. The E6700's thermal rating, or TDP, is 65W, while the X6800's is 75W. Fittingly, the QX6700's TDP is exactly twice that of the E6700 at 130W. That's quite enough heat production for a desktop processor, and Intel has already established a 130W thermal envelope for Pentium Extreme Edition CPUs that use this same LGA775 infrastructure.

In fact, the QX6700 should be compatible with many existing Core 2-compatible motherboards via nothing more than a BIOS update. Some mobo makers have already published compatibility lists for Kentsfield. Then again, Intel says previous revisions of its own D975XBX "BadAxe" mobo aren't designed for use with the QX6700, so nothing is certain. You'll want to check with the motherboard maker to ensure compatibility before taking the plunge.

Because the QX6700 is an Extreme Edition processor, it comes with a customarily robust price tag of $999 and an unlocked upper multiplier to facilitate easy overclocking. Intel also plans to introduce a less expensive Core 2 Quad Q6600 CPU at some point in the first quarter of next year. That product will run at 2.4GHz and have a TDP rating of 105W.

Quad-core's performance challenges
The Core 2 Extreme QX6700 may be the apex of awesomeness in processors today, but it does face some formidable performance challenges, both due to its own nature and because of external factors. As a multi-chip package, the QX6700 contains two copies of a relatively well-integrated dual-core design. The two cores on each chip share a 4MB L2 cache between them, complete with dynamic partitioning and the ability to hand off ownership of data from one core to the next. Unfortunately, the integration between the QX6700's two chips is less than ideal.

Although they occupy the same package, their only means of communication is the system's front-side bus. The two chips must coordinate to ensure the sanity of the contents of their respective L2 caches via this bus. That will sometimes mean writing modified data out of one chip's cache into main memory and then reading it back into the other chip's cache—a positively eternal operation in CPU time. Both chips use this same bus to talk with the rest of the system, including main memory and I/O devices. Also, the presence of three electrical loads on the bus—two CPU chips and the core-logic chipset's north bridge—complicates matters. Someone looking to overclock his system's FSB may find less success with a Core 2 Quad or QX6700 than with a standard-issue Core 2 Duo.

If all of that sounds complex, just wait until you dig into the software issues. In order to take advantage of multi-core processors, software applications must execute by means of multiple threads. Today, very few games and not many other applications are multithreaded. We do try to take advantage of multithreaded applications when possible in our CPU test suite, but that's more difficult to do for four cores than for two. Many of the early optimizations for multi-core processors only use two threads, so their performance benefits are fully realized on a dual-core CPU.

There are reasons for this situation. For instance, one of our test apps, the MP3 encoding program LAME MT, employs a technique called linear pipelining that processes a portion of its work one frame ahead of the main thread and then buffers the result for later use. This method uses only two threads and can't take advantage of more than two CPU cores, but it is relatively easy to program. LAME MT's author says of linear pipelining: "In general, this approach is highly recommended, for it is exponentially harder to debug a parallel application than a linear one." On a similar note, we have seen measurable performance gains in dual-core systems using graphics drivers that offload some vertex processing to a second thread, but Nvidia's drivers, at least, don't appear to benefit from the presence of more than two cores.

The thread scheduling mechanism in Windows presents another challenge for quad-core processors, because it doesn't always make the best decisions. During our testing, for example, we found that the Core 2 Extreme QX6700 was turning in substantially lower performance running the same single-threaded task—a POV-Ray scene render—than the like-clocked Core 2 Duo E6700. This behavior was consistent across multiple benchmark runs and a little bit puzzling, until we looked at the Windows Task Manager as this process ran. Turns out the rendering work was bouncing around across all four of the QX6700's cores, playing havoc with cache locality and the like.

For the most part, you can expect the Core 2 Extreme QX6700 to perform like a Core 2 Duo E6700 in applications that use only one or two threads, but the QX6700 may prove slower in some cases due to additional bus overhead or bad thread management in Windows. Of course, when applications use more than two threads or more than two apps are running at once, the QX6700 will pull the tab back and pop open a can of whupass. We have some applications like that in our test suite, so you can see quad-core's true potential.

That potential, by the way, will almost certainly be more fully realized by future applications, especially games. Software developers know that multi-core processors are the future, and high-profile game development houses have been working on game engines that use multiple threads to handle various tasks. Heck, they practically have to given that the Xbox 360 and the PlayStation 3 have multi-core CPUs. Doing this kind of thing well is by no means a trivial undertaking, but the general trajectory seems to involve spinning off threads for specific game elements like A.I., physics, rendering, and audio. Industry giants like Microsoft and Intel have been pouring resources into helping the conversion to multithreading happen, and I'm convinced that it will.

If you're not convinced, perhaps a couple of statements that Intel forwarded to us from key game developers will help. Here's Tim Sweeney, Founder and President of Epic Games:

Multi-core computing is the new standard for PC games, and we at Epic are thrilled to see Intel leading the industry forward with Core 2 Extreme. Its four high-performance CPU cores enable a new level of realism in games, with realistic physics simulation, character animation, and other computationally-intensive systems.
And here's Gabe Newell, President and co-founder of Valve:
Quad-core will change every aspect of PC gaming. It will change how we create our games, how we provision our service, and how we design our games. The scalability we've seen in graphics over the last few years will now extend to physics, AI, animation, and all the systems which are critical to moving beyond the era of pretty but dumb games.
I don't think these guys are just issuing blanket statements of support in order to play nice. That's not been their style, historically. In fact, we will have more coverage of the specifics of Valve's multithreading efforts very soon, so stay tuned for that.

Between now and when those next-generation game engines arrive, owners of quad-core processors will have to find other ways to take full advantage of their CPUs. The test results on the following pages offer numerous examples of applications that use four threads, and beyond that, there's always the prospect of really, really good multitasking. My initial reaction is that you don't need four cores for good multitasking. Despite frequent abuse, my current Athlon 64 X2-based desktop system rarely slows down, and when it does, available CPU time isn't the likely culprit. Then again, I sure wouldn't complain about having four cores at my beck and call.