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QPI, Integrated Memory, PCI Express, And LGA 1156

Intel Core i5 And Core i7: Intel’s Mainstream Magnum Opus
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The company won’t get specific about the changes made to its QuickPath Interconnect, the point-to-point interface first used to attach Core i7 to X58. But there is a QPI link inside of Lynnfield, serving as the glue between on-die PCI Express and the uncore. Intel has a lot more freedom with this technology now, since it doesn’t have to leave the die. Thus, the performance and timings are both reportedly better here than they were on Bloomfield.

This interconnect is important today, naturally, but will become a key performance enabler when Clarkdale launches next year. With graphics and memory control on the same piece of silicon, memory bandwidth—one of the biggest Achilles’ heels of integrated graphics designs—is delivered much more effectively. As a result, expect to see Intel adjusting the speed of its internal QPI link up or down to differentiate its Clarkdale SKUs.

Of course, none of that is really a concern with Lynnfield, which employs 16 lanes of integrated PCI Express 2.0 to interface with discrete GPUs. We’ve already spent considerable time comparing the performance of single- and dual- card configurations on P55 against X58, P45, and 790GX using 2.8 GHz processors. We've determined that there’s very little gain or loss resulting from Intel’s Lynnfield implementation. In other words, the integration of PCI Express is just about as close to transparent to end-users as possible. This is the way integration should work (though most of us have been conditioned to think it automatically leads to performance sacrifices).

Much more impactful is the licensing of SLI and CrossFire, which allows most P55-based motherboards to support either technology, just like X58. Of course, the difference is that P55 platforms are going to be significantly cheaper. More on the chipset shortly.

Finally, you have the processor’s integrated DDR3 memory controller, which is cut from three 64-bit channels in Bloomfield to two 64-bit channels with Lynnfield. Officially, Bloomfield is rated for up to DDR3-1066, yielding a theoretical maximum of 25.6 GB/s of bandwidth. We’ve all seen the LGA 1366-based Core i7s scale as high as DDR3-2133 though, so the official spec means very little to enthusiasts. In contrast, Lynnfield is rated for DDR3-1333 memory modules, offering up to 21.3 GB/s. For the most part, the loss of that third channel is made up for by an increase in attainable data signaling rate. We’ll put this theory to test in a couple of pages.

LGA 1156: More Socket Segmentation

The table below should look somewhat familiar; I used it back in February to chart the last nine years of desktop socket launches from Intel and AMD. With the debut of LGA 1156, the Intel column is looking pretty darned crowded.

Disruptive Socket Launches
Year
AMD
Intel
2001

Socket 478
2002


2003
Socket 754

2004
Socket 939
LGA 775
2005


2006
Socket AM2

(Intel launches Core 2 Duo, most motherboards need to be replaced)

2007


2008

LGA 1366
2009
(Socket AM3new processors work in old motherboards, but not the other way around)LGA 1156


I’m sure there will be a number of enthusiasts who’ve spent a lot of money on LGA 1366, X58, and Core i7. You'll see the performance of Lynnfield, compare the cost, look at the upgrade path to Clarkdale, and ask me why I couldn’t have given a more vocal heads-up about what was to come. To those folks, let me give you one hopefully-welcome piece of information: the upcoming Gulftown design (32nm, hexa-core, Hyper-Threading-enabled) will be a drop-in upgrade to your existing platform. Meanwhile, those waiting on Clarkdale have a dual-core design to look forward to—certainly not as sexy, unless you’re really itching for integrated graphics. 

What, then, was the impetus behind LGA 1156 less than a year after LGA 1366? And why couldn’t Intel integrate PCI Express on the LGA 1366 platform, too?

Right from the get-go, 1366 was designed as a DP interface for Intel’s Tylersburg platform. The fact that it surfaced first on the desktop is similar to how LGA 771 made a brief appearance in Intel’s Skull Trail gaming configuration. The difference is that motherboards based on X58 made their way under $200 and the cheapest processors dropping into the interface are less than $300. That’s not little money, to be sure. But it’s affordable enough to tempt the folks who know how fast an overclocked i7 machine can be.

LGA 1366 didn’t get PCI Express because it would have made the pin-out far too complex. Plus, the servers and workstations that now use the 5520 and 5500 chipsets need access to more than just 16 lanes of connectivity for storage controllers, professional graphics cards, and high-speed networking.

LGA 1156: the real mainstream interfaceLGA 1156: the real mainstream interface

Thus, LGA 1156 becomes the true successor to LGA 775, despite now-convoluted segmentation. It’s simpler than LGA 1366, its locking mechanism applies pressure more evenly on the CPU’s heat spreader, and it helps protect against EMI—particularly important when Clarkdale exposes on-chip graphics.

You can expect this processor interface to remain pervasive until the end of 2011, when LGA 1155 (planned launch in the first half of 2011) starts grabbing market share big time.

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