PCIe X16 Slot @ X4

[box o rocks]

I've read here and on other forums how adding a 2nd GFX card to a X16, X4 PCIe 2.0 system would drag down the performance due to the limited bandwidth the 2nd card would have with only 4 lanes. In fact, my friend tried it with his Biostar board and 2 HD 6850s and the results sucked. The results were no better than 1 card and sometimes lower.

Yet... why doesn't that happen with 3-way/4-way SLI or CF when the last card is almost always in a PCIe slot with 4 lanes?

 

[geekapproved]

Who says it doesn't??

 

[dudewitbow]

crossfire/sli acts different on some games. a 16x/4x crossfire will lose out in some performance. in some games, turning on crossfire/sli gives you negative scaling.

 

[Maxx_Power]

I've read here and on other forums how adding a 2nd GFX card to a X16, X4 PCIe 2.0 system would drag down the performance due to the limited bandwidth the 2nd card would have with only 4 lanes. In fact, my friend tried it with his Biostar board and 2 HD 6850s and the results sucked. The results were no better than 1 card and sometimes lower.

Yet... why doesn't that happen with 3-way/4-way SLI or CF when the last card is almost always in a PCIe slot with 4 lanes?

My experience exactly when I was curious about this. Either get a LGA2011 for proper lane widths, or get a PCI-E 3.0 CPU+board so you can split lanes into 2xPCI-E 8x (gen 3.0) each slot of which have essentially the same bandwidth as PCI-E 2.0 16x slot. Less overhead, and higher transaction rate.

That and microstutter plus negative scaling on a lot of non-trendy games (and newest ones) made me nail the final nail in the coffin of multi-GPU setups.

Of course, if you have the money, and all the games you play or plan to play are compatible, no one is saying don't try it.

 

[box o rocks]

crossfire/sli acts different on some games. a 16x/4x crossfire will lose out in some performance. in some games, turning on crossfire/sli gives you negative scaling.

We found that out!

 

[box o rocks]

get a PCI-E 3.0 CPU+board so you can split lanes into 2xPCI-E 8x (gen 3.0) each slot of which have essentially the same bandwidth as PCI-E 2.0 16x slot. Less overhead, and higher transaction rate.

You're saying that because IB has more lanes, 3-way would be x8, x8, x8?

 

[Maxx_Power]

You're saying that because IB has more lanes, 3-way would be x8, x8, x8?

IB has Gen 3 lanes. Each lane in IB is twice as fast as a lane in SB or earlier.

So it goes like this,

Each lane in PCI-E is

Gen 3 = 2x Gen 2 = 2x Gen 1.

So, 16 lanes from IB, can be split into 8x, 8x Gen 3, each of which is as fast as 16x, 16x Gen 2. This is optimal for 2-way, for 3 or more ways, you still have to go to LGA2011.

You need a board that supports Gen 3, by means of adoption of the Gen 3 PCI-E switch chips (usually PLX branded). This was what the fuss was all about with the Z68 boards and MSI vs. Gigabyte on who actually has Gen 3 support due to the switching chips.

 

[box o rocks]

@ maxx_power
Thanks for the info. I didn't know that. If you find time, I'd appreciate a link to a PC site that this is reviewed in.

 

[Maxx_Power]

@ maxx_power
Thanks for the info. I didn't know that. If you find time, I'd appreciate a link to a PC site that this is reviewed in.

This is the best latest one:

http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/

It includes extensive benchmark results, a brief introduction to PCI-E bandwidth (1.x, 2.x, 3.0), and all relevant conclusions.

Off the top of my head, I don't remember seeing any SLI-Crossfire scalings with PCI-E 3.0, but here is a quickie:

 

[box o rocks]

This is the best latest one:

http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/

It includes extensive benchmark results, a brief introduction to PCI-E bandwidth (1.x, 2.x, 3.0), and all relevant conclusions.

Off the top of my head, I don't remember seeing any SLI-Crossfire scalings with PCI-E 3.0, but here is a quickie:

http://i119.photobucket.com/albums/o139/callsign_vega/PCI-ETests.jpg

Holy Crap! 4-way SLI with GTX 680s! I could buy a car for what that rig must cost...
But much appreciated. I will check out the article. Man, BF3 across triple monitors with that kind of frame rate... wow.

 

[Maxx_Power]

Holy Crap! 4-way SLI with GTX 680s! I could buy a car for what that rig must cost...
But much appreciated. I will check out the article. Man, BF3 across triple monitors with that kind of frame rate... wow.

Yeah... Gaming was/is never this important to me, I'd keep my car.