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Crossfire X8 + X8 Versus X16 + X16

Crossfire Meets PCI Express 2.0
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The X38 chipset allows two graphics cards running in a dual-card configuration to each use an x16 connection. Since this Intel chipset only supports a dual-card setup consisting of ATI cards, our testing is limited to the fastest of ATI's current crop of cards.

Also, we limited our benchmarks to the resolutions at which Crossfire configurations showed pronounced performance gains on our old 975BX2 motherboard using a 2x8 configuration. In all, our benchmark suite usually consists of 35 test resolutions, but we're working with a reduced sample of only 13 here.

Call of Duty 4
1280x1024, 0xAA, Trilinear, max. Quality
1680x1050, 0xAA, Trilinear, max. Quality
1920x1200, 0xAA, Trilinear, max. Quality
1280x1024, 4xAA, 8xAF, max. Quality
1680x1050, 4xAA, 8xAF, max. Quality
1920x1200, 4xAA, 8xAF, max. Quality
Doom 3
1600x1200, 4xAA, 8xAF, Ultra Quality
Half Life 2 Episode 2
1280x1024, 4xAA, 8xAF, Very High Quality
1680x1050, 4xAA, 8xAF, Very High Quality
1920x1200, 4xAA, 8xAF, Very High Quality
World in Conflict
1280x1024, 0xAA, Trilinear, Very High Quality
1680x1050, 0xAA, Trilinear, Very High Quality
1920x1200, 0xAA, Trilinear, Very High Quality

In Crossfire mode, both the Radeon HD3870 and the HD2900 XT show a performance improvement of 12 percent. The Radeon HD3850 benefits the most, gaining 15 percent.

Overall Performance - Crossfire
PCIe x8x8 vs x16x16
fps Percent
HD2900 XT (512 MB) CF PCIe 1.0a x8x8 1190.1 100.0
HD2900 XT (512 MB) CF PCIe 2.0 x16x16 1335.1 112.2
HD3870 (512 MB) OC CF PCIe 1.0a x8x8 1163.8 100.0
HD3870 (512 MB) OC CF PCIe 2.0 x16x16 1312.8 112.8
HD3850 (256 MB) OC CF PCIe 1.0a x8x8 981.4 100.0
HD3850 (256 MB) OC CF PCIe 2.0 x16x16 1128.6 115.0

Before you denounce our previous tests as being unfair towards ATI, let's put those results in perspective. The performance gains of between 12 and 15 percent only apply to 13 of 35 tests. Factoring this change into the overall result of all 35 resolutions, we are left with a performance boost of between 6 and 7.7 percent. Finally, we have to take a realistic look at where this performance boost actually manifests itself, by looking at the individual results, which you can find in our benchmark charts in this article.

We saw the biggest performance increase of nearly 20 percent in Call of Duty 4, with a Radeon HD3850 Crossfire configuration. On the other hand, that game already ran silky smooth even on an x8 link. We can only really call the 1920 resolution with FSAA a real win on the HD3850, with the frame rate going from 53 to 63 fps. The rest of the resolutions already easily ran at more than 60 fps or 100 fps on all three dual-card configurations to begin with. Nonetheless, we did clock a new record: the Radeon HD2900 XT delivered 215.6 fps in Crossfire mode, where the Geforce 8800 GTS 512 reached "only" 176.7 fps on a 680i-based motherboard.

In Doom 3, performance increased by 5 percent, going from 105 to 110 fps. Half Life 2 Episode 2 saw a performance increase of 9 percent on the new HD38x0 cards, going from 64 to 71 fps or 75 to 80 fps. In World in Conflict, the performance improvement was measurable, though not tangible: 1 to 3 fps faster.

Crossfire Individual Performance - Call of Duty 4
PCIe x8x8 vs x16x16
fps Percent
HD2900 XT (512 MB) CF PCIe 1.0a x8x8 779.2 100.0
HD2900 XT (512 MB) CF PCIe 2.0 x16x16 904.8 116.1
HD3870 (512 MB) OC CF PCIe 1.0a x8x8 747.3 100.0
HD3870 (512 MB) OC CF PCIe 2.0 x16x16 870.9 116.5
HD3850 (256 MB) OC CF PCIe 1.0a x8x8 593.0 100.0
HD3850 (256 MB) OC CF PCIe 2.0 x16x16 710.5 119.8
Crossfire Individual Performance - Doom 3
PCIe x8x8 vs x16x16
fps Percent
HD2900 XT (512 MB) CF PCIe 1.0a x8x8 107.7 100.0
HD2900 XT (512 MB) CF PCIe 2.0 x16x16 112.9 104.8
HD3870 (512 MB) OC CF PCIe 1.0a x8x8 106.7 100.0
HD3870 (512 MB) OC CF PCIe 2.0 x16x16 112.6 105.5
HD3850 (256 MB) OC CF PCIe 1.0a x8x8 104.1 100.0
HD3850 (256 MB) OC CF PCIe 2.0 x16x16 109.0 104.7
Individual Crossfire Performance - Half Life 2 Ep. 2
PCIe x8x8 vs x16x16
fps Percent
HD2900 XT (512 MB) CF PCIe 1.0a x8x8 223.2 100.0
HD2900 XT (512 MB) CF PCIe 2.0 x16x16 234.4 105.0
HD3870 (512 MB) OC CF PCIe 1.0a x8x8 219.8 100.0
HD3870 (512 MB) OC CF PCIe 2.0 x16x16 239.3 108.9
HD3850 (256 MB) OC CF PCIe 1.0a x8x8 212.3 100.0
HD3850 (256 MB) OC CF PCIe 2.0 x16x16 231.1 108.9
Individual Crossfire Performance - World in Conflict
PCIe x8x8 vs x16x16
fps Percent
HD2900 XT (512 MB) CF PCIe 1.0a x8x8 80.0 100.0
HD2900 XT (512 MB) CF PCIe 2.0 x16x16 83.0 103.8
HD3870 (512 MB) OC CF PCIe 1.0a x8x8 90.0 100.0
HD3870 (512 MB) OC CF PCIe 2.0 x16x16 90.0 100.0
HD3850 (256 MB) OC CF PCIe 1.0a x8x8 72.0 100.0
HD3850 (256 MB) OC CF PCIe 2.0 x16x16 78.0 108.3

Overall, the change in platforms results in a speedup of only a few percent. Games that ran smoothly before run a little faster, while slower games such as Crysis or Flight Simulator X don't yet benefit from Crossfire optimizations in the WHQL drivers. Currently, going from x8 to x16 for Crossfire really only offers a tangible payoff in higher resolutions. At the highest resolution of 1920x1200 pixels, only a single test enjoyed a really meaningful speedup as a result of the faster interface, going from 53 to 63 fps. The remaining resolutions were already running smoothly at more than 70 or 100 fps, so any improvements here only help the overall performance rating, not real-world playability.

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    ART-T , April 18, 2008 9:26 PM
    I read an article that was comparing PCIe cards into slots that were only 8x and 4x.

    I was wondering if that means you can buy PCIe 2.0 cards and put them in a x16 slot (and still work; but with degraded performance).