Will a single VGA in a PCIe 2.0 x16 (x4 mode) slot perform the same as in a PCIe 2.0 x16 one?

Question

I have an ASUS P8P67-M REV 3.0 which has the following PCIe 2.0 slots:

• 1 x PCIe 2.0 x16 (blue)
• 1 x PCIe 2.0 x16 (x4 mode, black)

I've just bought a new VGA (GTX 970), which is huge, and when inserted in the blue PCIe socket I cannot plug anything into the SATA III ports.

I could move the card to the black socket, which is at the botton of the motherboard and doesn't block anything.

Will the VGA perform the same whether is inserted in the normal PCIe 2.0 x16 or the x4 mode one?

What does that x4 really mean?

Answer 1

No, sorry.

The bandwidth available from a PCIe slot is determined by the PCIe version (in this case 2.0) and the number of "lanes" it has. Each "lane" in PCIe 2.0 offers throughput of up to 500 MB/s.

Your "2.0 x16 (blue)" slot has 16 PCIe lanes. That permits throughput of up to 8 GB/s. (That's 16 times 500 MB/s.)

Your "x16 (x4 mode, black)" slot is the size of the "2.0 x16 (blue)" slot but it only has four PCIe "lanes" in it. That's what "x4 mode" means. So, even though it is able to physically accept x16 cards, its throughput will be limited to 2 GB/s.

Now... depending on what you're doing with the card, that might not matter. But if you're running high-performance games, I expect you'd notice the difference. I doubt it would be as severe as a reduction in frame rate to just 25%, but a reduction to 50% wouldn't be unexpected.

I should probably mention that you are already not getting all the performance from that card that you're paying for, because it is a PCIe 3.0 card. PCIe 3.0 provides almost twice the throughput per lane as PCIe 2.0 (985 MB/s per lane vs. 500). The tests you may have seen for the card's performance assume that you're using it in a PCIe 3.0 slot. So it will not run as fast as indicated in those reports in a 2.0 slot. Most video cards in most applications are not actually using all the bandwidth PCIe offers, so the difference may not matter much, but it's something to keep in mind. The only way to fix that is to get a motherboard that has PCIe 3.0. (While you're at it you could get one in which a long PCIe x16 card won't block the SATA ports. ;) )