A quick look at chipset PCI Express performance

The P67 brings PCIe 2.0 up to full speed
— 10:23 AM on January 14, 2011

In addition to support for Sandy Bridge CPUs, Intel's latest P67 Express core-logic chipset brings with it two major improvements over the old P55: a couple of 6Gbps Serial ATA ports and full-speed PCI Express 2.0 lanes throughout. Those features have been present in AMD chipsets since the SB850 south bridge debuted some ten months ago, making Intel more than a little late to the party. At least on the SATA front, the wait was worth it. While comparing four mid-range P67 boards, we discovered that Intel's 6Gbps Serial ATA ports are faster than AMD's.

So, what about the P67's PCI Express lanes? Those familiar with the P55 will remember that it, too, has PCIe lanes claiming second-generation status. But those lanes run at half speed and offer no more bandwidth than gen-one PCIe links. The P67's PCI Express 2.0 lanes are cranked up to the standard's full 5.0GT/s data rate, setting up a three-way comparison with the P55 and AMD's SB850.

We had planned to incorporate a new batch of PCIe 2.0 performance tests in our initial motherboard round-up. However, it took a little time to find an appropriate peripheral card with a PCIe 2.0 x1 interface. And then it took a little longer for that card to arrive at the Benchmarking Sweatshop.

Incidentally, there are plenty of PCIe peripherals available these days—just few that profess to support second-generation PCI Express. Most of the PCIe solid-state drives use x4 interfaces that make isolating a single lane impossible, so we settled for the next best thing: a RAID card attached to a couple of 2.5" SSDs.

The only suitable candidate we could find was Syba's SY-PEX40032, which situates dual 6Gbps Serial ATA RAID ports behind a PCIe 2.0 x1 interface and sells for $40 at Newegg. You've probably never heard of Syba, and neither had we. The card uses the Marvell 88SE9128 6Gbps SATA RAID controller we've seen on just about every enthusiast-oriented motherboard to be sold in the last year, though. With a couple of SandForce-based Agility 2 and Vertex 2 solid-state drives from OCZ arranged in a striped RAID 0 array, we had ourselves nice little PCIe 2.0 SSD. We've seen each of those solid-state drives push transfer rates of nearly 200MB/s on their own. Teaming them in RAID 0 should be enough to exceed the 250MB/s of one-way bandwidth offered by a first-generation PCI Express lane. PCIe 2.0 offers 500MB/s of bandwidth per lane, and this array is our best shot of exploiting it.

Before creating our RAID 0 array, we secure-erased the SSDs to ensure optimal performance. We settled on a stripe size of 32KB (64KB was the only other option in the Marvell BIOS) for the array because it offered the best performance in the benchmarks we'd selected for testing.

To represent Intel's P67 Express chipset, we called on the company's own DP67BG motherboard using the same system configuration as in our mobo round-up. The Asus 890GX motherboard from that article sat in to represent AMD's latest south-bridge PCIe implementation. However, we had to find a different P55 board to get a direct path to the chipset's built-in PCIe lanes. Most enthusiast-oriented P55 boards employ switch chips that share and distribute the P55's PCIe lanes between multiple expansion slots and onboard peripherals. That kind of middleman isn't terribly helpful when you're trying to isolate a single PCIe lane, so I dug through the mess of boxes in my office and unearthed a MSI P55-GD65 motherboard with an x1 slot leading straight to the P55 PCH.

First up: HD Tune, which we've been using to test hard drive and SSD performance in storage reviews for nearly a year now. We're sticking with read speed tests to avoid writing to the array and dodge the block-rewrite penalty inherent to flash-based storage. Unfortunately, TRIM commands can't yet be passed through a RAID controller.

The P67 Express' PCIe 2.0 lanes allow our SSD RAID array to achieve much higher read speeds than when it's attached to the AMD chipset. That result is a little unexpected given that the 890GX's SB850 south bridge claims to offer a full-speed PCIe 2.0 implementation. In fact, the AMD platform trails the P67 by a much larger margin than it enjoys over the P55, which has a built-in handicap.

HD Tach produces similar results. The P67 has a substantial lead over the 890GX, while the P55 brings up the rear.

Our SSD array hits higher speeds here than in HD Tune. Even so, we're still a ways off PCIe 2.0's maximum per-lane bandwidth of 500MB/s in each direction.

Interestingly, HD Tach reveals higher CPU utilization for the AMD chipset. We wouldn't normally make much of deltas in CPU utilization between platforms using different processors. In this case, however, the gap is much larger than we'd expect between a six-core Phenom II and the quad-core, Hyper-Threading-equipped Core i7 CPUs used on the Intel boards.

For what it's worth, when we ran the same tests on a P55 board that employs a PCIe switch chip, we saw even higher transfer rates than on the P67 Express. Motherboard makers appear to have done a good job of working around the P55's limited PCIe bandwidth. We should also point out that the RAID 0 performance of Marvell's 88SE9128 is pretty poor considering what just one of those SSDs can do on its own. With each drive capable of pushing reads at 190MB/s, some performance is clearly being left on the table.

Regardless, the Marvell controller is a popular solution that performs substantially better when connected to the PCI Express lanes attached to Intel's P67 Express chipset than it does on competing platforms. The P55 is much slower because its PCIe 2.0 lanes have been dialed back to half speed. However, the SB850 doesn't have that excuse.

Perhaps we shouldn't be surprised. AMD inherited a history of chipset issues when it acquired ATI, while Intel's core-logic record is nearly spotless through a multitude of generations. The P67 Express looks poised to live up to that reputation.TR

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