PCIE3 @ x8 mode

A2BDriver · 25 Jun 2012 at 08:58

Ok, am looking at maybe updating to Z77 chipset, and looking for sli support.

I see a few motherboards that say 2 x PCI Express 3.0 x16 slots (PCIE1/PCIE3: single at x16 (PCIE1) / x8 (PCIE3) or dual at x8/x8 mode)

and that got me thinking, what's the point of having pcie3 if its only running at x8, not that I want to get in the way of technology an all, but I see this as pointless? or do I not understand properly.

Any recommended sli boards I should be looking at.

Mirmo · 25 Jun 2012 at 10:08

A PCI-E 3 slot running at 8x is the same as a PCI-E 2 running at 16x so in SLI on that board you'll be running at the same speed as 2 PCI-E 2x16. This won't be a bottleneck at all as you're gfx card doesn't saturate a PCI-E 2 x16 as it is.

siege2 · 25 Jun 2012 at 10:17

From here it states

http://en.wikipedia.org/wiki/PCI_Express

v2.x: 500 MB/s (5 GT/s)
v3.0: 1 GB/s (8 GT/s)

What you would need is M/B with full PCI-E 3 * 2 specs.

I Have a AMD 7970 so my gpu would get the benifet from it, are u going to use a PCI-E 3 GPU etc ? otherwise PCI-E 2 would do you.

Kenneth.

A2BDriver · 25 Jun 2012 at 10:37

So its not really at the consumer level yet? its mainly for solid state drives and raid controller cards.

I would be looking at adding a second gtx680

eddyr · 25 Jun 2012 at 12:18

I'd say there's no necessity of pcie 3 for a single isolated typical consumer level application but it does also allow for more add-in cards at the same bandwith so progression is natural to the new standard even if there is no direct need.

mmj_uk · 25 Jun 2012 at 12:45

You will have nearly twice the bandwidth with PCI-E 3.0 8x/8x compared to PCI 2.0 8x/8x, that's the point.

The reason they are only running at 8x is more of a limitation of the LGA1155 platform, the CPU's only have a total of 16 PCI-E lanes built into them so if you want to use two graphics cards then they have to use 8 lanes each.

8 Pack · 26 Jun 2012 at 13:52

4x is enough for all current graphics cards it seems. The world record for unigen with 3 x 7970 has two of them at 4x on Maximus V Formula board.

Unigen is a graphics card limiting bench too.

Dirtyganker · 26 Jun 2012 at 15:42

Both what mmj_uk and 8 Pack said is correct. Remember that PCIE 3 is a ratified standard by the PCIE SIG and is for use across a number of different platforms, not just Z77 and Ivybridge. Z77 and IB obviously have the limitation of only x16 on one slot or x8/x8 because thats all that platform is capable of offering natively, whereas on X79 and Sandybridge-E you have can x16/x16.

The reality is that even at x8 on PCIE 2 the GPU is still the bottleneck, not the bus. Hence this is the reason why SLI/Xfire is able to scale and two cards give better performance than one. If the bus was saturated then adding additional cards would do very little, which we know is not the case. Don't fear x8/x8. Start worrying when you can look at your GPU utilisation meters and see that it's not running at 100% when running benchmarks, thats when you know your card is being under utilised and something is getting in the way.

It's definately the way forward and seems to be maturing as an interconnect standard as time goes on.