PCIe x8 vs. PCIe x16

PCI express expansion slots vary in both physical size and the amount of bandwidth they provide. The really long slots are called x16 slots, as they provide 16 lanes of PCIe bandwidth, and that’s where our video cards go, for obvious reasons. Almost all of the top slots in a motherboard (those closest to the CPU) are x16, but sometimes those 16 lanes are divided between two slots, so what might look like a x16 slot is actually a x8 slot. The tricky part is that sometimes the slots below the top slot only offer eight lanes of PCIe bandwidth, and sometimes people need to skip that top slot because their CPU cooler is in the way or water cooling tubes are coming out of a radiator in that location. Or you might be running a dual-card setup, and if you use a x8 slot for one card, it will force the x16 slot to run at x8 speeds. Here’s the question: Since a x16 slot provides 3.2GB/s of bandwidth in one direction, and a x8 slot pumps 1.6GB/s, is your performance hobbled by running at x8?

THE TEST: We wedged a GTX Titan into first a x16 slot and then a x8 slot on our Asus P9X79 motherboard and ran our gaming tests in order to compare the difference.

THE RESULTS: We were surprised by these results, which show x16 to be a clear winner. Sure, it seems obvious, but we didn’t think even current GPUs were saturating the x8 inter­face, but apparently they are, so this is an easy win for x16.

BENCHMARKS

GTX Titan PCIe x16

GTX Titan PCIe x8

3DMark Fire Strike

9,471

9,426

Catzilla (Tiger) Beta

7,921

7,095

Unigine Heaven 4.0 (fps)

40

36

Crysis 3 (fps)

32

37

Shogun 2 (fps)

64

56

Far Cry 3 (fps)

43

39

Metro: Last Light (fps)

25

22

Tomb Raider (fps)

25

23

Battlefield 3 (fps)

57

50

Tests performed on an Asus P9X79 Deluxe motherboard.

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