SLI + PCIe + Link Lanes?

By mikeadams1137 ·
Hi all.

I have a question in regards to the SLI features I will be enabling in my new computer system. This post will consist of general components, researched information from default manufacture websites, and research done on my own. Onto the show...

Motherboard: [CrossFireX/SLI] GigaByte GA-Z68MA-D2H-B3 Intel Z68 Chipset DDR3 mATX MB w/ Lucid Virtu Intel Smart Response Technology & 7.1 HD Audio, GbLAN, USB3.0, 2x SATA-III RAID, 2 Gen2 PCIe, 1 PCIe X4 & 1 PCIe X1

* Supports Core i7/Core i5/ Core i3 processors
* Intel Z68 Express Chipset
* Support Dual channel memory architecture for DDR3 2133/1866/1600/1333/1066 MHz
* High Definition Audio Realtek 2/4/5.1/7.1 channel with S/PDIF Out
* 1 x RJ45 LAN Realtek RTL8111E chip (10/100/1000 Mbit)
* 1 x PCI Express x16 slot @ x16, 1 x PCI Express x16 slot @ x8, 1 x PCI Express x1 slots
* Supports both ATI CrossFireX and NVIDIA SLI technology
* 2x SATA 6Gb/s and 4x SATA 3Gb/s
* 12 USB 2.0/1.1 and 2 USB 3.0/2.0 ports
* 2x IEEE 1394a ports
* 1 x DVI-D port supporting a maximum resolution of 19201200
* 1 x HDMI supporting a maximum resolution of 19201200
* 1 x optical S/PDIF Out connector
* 1 x D-Sub port

Processor: Intel Core i5-2500K 3.30 GHz 6M Intel Smart Cache LGA1155

Memory: 8GB (2GBx4) DDR3/1600MHz Dual Channel Memory (Corsair XMS Gaming Memory with Heat Spreader)

Cooling: Asetek 510LC Liquid Cooling System 120MM Radiator & Fan (Enhanced Cooling Performance + Extreme Silent at 20dBA) (Dual Standard 120MM Fans (Push-Pull)

Video Card 1: NVIDIA GeForce GTX 560 Ti 1GB 16X PCIe Video Card [+256] (EVGA Superclocked)
Video Card 2: NVIDIA GeForce GTX 560 Ti 1GB 16X PCIe Video Card [+256] (EVGA Superclocked)

This is the first computer I've built since the AMD Barton 2500+ unlocked processor days and PC3200 Memory. :) AGP was the norm, and PCIe was just becoming feasible for the consumer base.

My main concern is with the motherboard. The motherboard has 2 PCIe x 16 Gen 2 slots ... one would assume that the aforementioned video cards will perform just fine. The motherboard is Z68 chip set and does have SLI support. In regards to PCI Express, the x factor represents the amount of lanes transmitting data in 8 bit parities. (2 by 2 method).

My question: If one PCIe slot is set to 16x @ 16x and the other PCIe slot is set to 16x @ 8x as the specifications of the motherboard indicate, I am assuming the SLI feature of the dual video cards will operate at 16x @ 8x speeds. Is this normal? Will this cause performance issues? I attempted to find a motherboard that had (2) PCIe 16x @ 16x, and for the highest end motherboard, I could not find one, $350+ ... Perhaps I am just incorrect in my understanding of how it all works, perhaps SLI support 16x @ 8x is actually normal. I just don't have the experience, your advice and input is appreciated.


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All Answers

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Well firstly I wouldn't be looking at any M'Board

by OH Smeg Moderator In reply to SLI + PCIe + Link Lanes?

With On Board Video as that is just the worst of everything possible . But saying that if you look on the Gigabyte Site you will see under Multigraphics Technology

1.Support for AMD CrossFireX??? technology
* The PCIEX16 slot operates at up to x4 mode when AMD CrossFireX??? is enabled.

So 'm assuming that when in Crossfire Mode the PCI E Slots only run at 4X not 16 bX.


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Reponse To Answer

by mikeadams1137 In reply to Well firstly I wouldn't b ...

Col, a bit confused here...I received a pretty good answer from the Nvidia Devs, which I will copy to here @ Geforce.com

AMD CrossFireX Technology only operates with ATI based video cards .. Nvidia utilizes SLI technology... not sure about the on-board video thing, I don't remember mentioning it. Unfortunately, with my current deployed status I cannot access Gigabyte's website.

This was posted to me on Geforce's website from one of their friendly forum technicians. I'll re-post it here for all to see, great information if you understand my question, and then her answer.

*** Begin Geforce.com Rep. Response ***

With current Sandy Bridge CPUs and motherboard chipsets, this setup is normal. There are not enough lanes available to supply two PCIe slots with the full compliment of 16 lanes each and still leave enough for peripheral hardware such as SATA and USB controllers, etc.. So, there is a switching arrangement that diverts 8 of the lanes from the primary slot to the secondary when a card is detected in that secondary slot. There are some (expensive) boards available with nVidia nF200 chips on them to provide additional PCIe lanes. But you don't need one.

In practice, with current graphics cards, the effect of this on performance is minimal. Only the very fastest cards saturate 8 lanes. There are some articles about on the web if you search. So have a read if your interested enough. Unfortunately, most of them do not address the SLI environment and only study the effect on single cards.

So, I have done some testing on this myself. With my motherboard, by moving the cards around, I can run two cards (in SLI) at either x16/x16 or x8/x8. The performance hit varies from game to game, with some suffering no measurable loss at all. The worst degradation I saw was about 4%, but the majority of games I tested only lost around 1 - 2% of framerate with both cards running at x8. Although, I didn't investigate myself, other studies I have read indicate that the effect gets worse at higher resolutions, such as 2560 x 1600 and above.

This is with GTX470s, which are fairly high-end cards. In fact, the GTX560Tis you are looking at are the direct replacements for the 470s and will give very similar performance. In fact, with their lower amount of RAM, they may be less affected.

So, you don't need to worry about this. The situation will be changing shortly. The next release of Intel CPUs and chipsets will have (a lot) more PCIe lanes between them and there should be enough for three full x16 slots. In addition, the next generation of graphics cards will probably support PCIe 3.0, which runs at double the speed of PCIe 2.x, so (on supporting motherboards) there will be more bandwidth than you can shake a stick at.


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Yep according to that the nVidia Runs at 8X in SLI Mode

by OH Smeg Moderator In reply to SLI + PCIe + Link Lanes?

On both PCI X Sockets and doesn't take much of a performance hit.

But the interesting thing I found here was the reference to new Chip Sets coming from Intel The situation will be changing shortly. The next release of Intel CPUs and chip sets will have (a lot) more PCIe lanes between them and there should be enough for three full x16 slots

It very well may pay to wait a little while till the new Chip Sets from Intel become available. It's about time for them to hit the streets maybe mid January if you can wait that long anyway.

I have not seen any Notices from Intel about these so other than the Normal Cycle which comes due in January they could be sooner.

The Gigabyte M'Board you mentioned has On Board Video which I find problematic to say the least and that's why I added the bit about On Board Video. These M'Boards have their own Video Chip Set and while OK they are not in the same league as a Dedicated Video Card/s and can cause conflicts with other hardware that you may want to plug in latter.


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