PCIe X16 Slot @ X4

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?
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  1. Who says it doesn't??
  2. 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.
  3. box o rocks said:
    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.
  4. dudewitbow said:
    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!
  5. Maxx_Power said:
    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?
  6. box o rocks said:
    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.
  7. @ 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.
  8. box o rocks said:
    @ 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:

  9. Maxx_Power said:
    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.
  10. box o rocks said:
    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. ;)
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