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Understanding PCI-e 16x

Hello,

I am trying to build a budget computer... Intel i5. Looking for a motherboard. I want to be able to run 2 video cards (not for gaming, for number crunching)

http://www.hardwaresecrets.com/article/MSI-H57M-ED65-Motherboard/908/2

I am trying to understand why the boards are advertised as having 2 16x slots, when the second slot is only 4x?

Will having a second card in there be slower performing than the first? (remember, not gaming, just using GPU support for distributed computing projects, etc)
7 answers Last reply Best Answer
More about understanding
  1. That's the speed it runs as even though it accepts 16 speed cards.

    Pcie can be put in any pcie slot it fits but that doesn't mean it will run at that speed.
  2. The slot is physically a 16x slot which means that it has the clearance to take a full sized PCI-E card while many 1x and 4x slots have something put immediately behind the slot that would interfere with the pins of a 16x card being put in that slot. It is electrically only a 4x slot since the CPU has a limited number of PCI-E lanes to work with.

    A card in the 4x slot will be slower than the same card in the 16x slot, for a GTX 480 it was about a 20% hit in performance, for a weaker card it will be a smaller percentage.
    http://www.tomshardware.com/reviews/pcie-geforce-gtx-480-x16-x8-x4,2696-9.html

    If you are just going to be using it for number crunching i doubt you will see a very significant difference in performance between the two, you probably wont end up maxing out the bandwidth.
  3. hunter315 said:
    The slot is physically a 16x slot which means that it has the clearance to take a full sized PCI-E card while many 1x and 4x slots have something put immediately behind the slot that would interfere with the pins of a 16x card being put in that slot. It is electrically only a 4x slot since the CPU has a limited number of PCI-E lanes to work with.

    A card in the 4x slot will be slower than the same card in the 16x slot, for a GTX 480 it was about a 20% hit in performance, for a weaker card it will be a smaller percentage.
    http://www.tomshardware.com/reviews/pcie-geforce-gtx-480-x16-x8-x4,2696-9.html

    If you are just going to be using it for number crunching i doubt you will see a very significant difference in performance between the two, you probably wont end up maxing out the bandwidth.



    That's in line with what I was thinking. Thanks for the help. I guess it's a safe assumption that a board with 2 true 16x slots is going to be very very expensive?
  4. Best answer
    Actually, its not existent for i3 and i5 CPUs, since their PCI-E lanes are determined by the CPU and the CPU only has 16 plus a few provided by the northbridge, you cant have better than 8x/8x and that would be more commonly found on a P55 board. If you want 16x/16x then you would need an X58 board with an i7 9xx or an AMD 890FX or 790FX chipset.
  5. Cool. Only about 40 bucks more for something like that. I think this thread can be closed. Woots.
  6. Best answer selected by decoy5657.
  7. This topic has been closed by Mousemonkey
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