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Does HD 6990 need PCI-E 16x?

Discussion in 'AMD / ATI' started by Yo_Wattup, Jan 27, 2012.

  1. Yo_Wattup New Member

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    I'm thinking of buying a 6990 to go with my 6950 but I've got the z68 chipset which only allows for crossfire in 8x mode. So my question is do I need the 16x when in CFX like the SB-e motherboards? Or will the 6990 reach its full potential at 8x?

    Thanks!
     
  2. Outback Bronze

    Outback Bronze

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    Your probably looking at a 2-3% drop if any at all. Youll b sweet!
     
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  3. micropage7

    micropage7

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    16x is better, but 8x is fine
    but if you wanna max it out you need to run it on 16x and your pciex need to matched to the graphic card pciex version, like pciex 2.0 slot would better paired with graphic card that use pciex 2.0
     
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  4. Yo_Wattup New Member

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    So what if I overclock my CPU's BCLK speed (say from 100 -> 104 which is about the most you can go with sandy bridge) therefore overclocking PCIe lanes? Would it make any difference at all?
     
  5. crazyeyesreaper

    crazyeyesreaper Chief Broken Rig

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    thing to keep in mind is 6990 is dual gpu

    you are dropping the 6990 to 4x per GPU die on chip and 8x for the 6950, this will starve bandwidth most people quoting performance drops are thinking of W1zzards review testing 1 GPU and the performance hit that comes with diminishing PCIE lanes,

    3 gpus on 2 x8 links will hurt performance more then some here may think due to a fairly sizable loss in bandwidth, as you add more GPUs communication between them takes more and more bandwidth,
     
  6. BarbaricSoul

    BarbaricSoul

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    also, three gpus yeild very little performance gains over two gpus. You'd be better off selling the 6950 and recovering some of the cost of the 6990 than trying to run all three cards IMHO.
     
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  7. crazyeyesreaper

    crazyeyesreaper Chief Broken Rig

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    true but 3 gpus does remove micro stutter almost completely in every game that benefits from dual gpu, which for some is worth considering,


    that said performance drop at x4 x4 x8 per gpu will have a 10% performance impact compared to 3 gpus having x8 x8 x8
     
  8. BarbaricSoul

    BarbaricSoul

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    Did not know that, will keep it in mind in the future.
     
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  9. crazyeyesreaper

    crazyeyesreaper Chief Broken Rig

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    Last edited: Jan 27, 2012
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  10. Yo_Wattup New Member

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    crazyeyesreaper - that's some interesting links you posted.. i was thinking about 6950 x 6970, but now I'm definitely going with the 6990. thanks.
     
  11. radrok

    radrok

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    Linus of NCIX made a review about PCIe lanes on HD6990, I remember that the difference was negligible.
    Also I am running one of the 6990s on PCIe 8x @ 2.0 because there are not enough lanes on X58
    I'll try to link that video when I come back, on phone atm would be tricky to search yt :)
     
  12. JrRacinFan

    JrRacinFan Served 5k and counting ...

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    Just curious; Why go the 6990 at all? Why not sell your current 6950 including the funds you were going to use towards the 6990 and put it towards a 79xx?
     
  13. radrok

    radrok

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    Here it is : http://www.youtube.com/watch?v=rSfifE2Domo

    I agree
     
  14. crazyeyesreaper

    crazyeyesreaper Chief Broken Rig

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    and yes its 1 6990

    your forgeting the 3rd GPU,

    you do realize the overhead when it comes to keeping all GPUs in sync increases greatly with each gpu added, your limiting each GPU to 4x4 on the 6990 which isnt terrible but now add a 3rd gpu at 8x that still has to use both crossfire bridges and PCIe bandwidth to communicate with the 6990s 2x GPUs, along with sync the vram to contain all data,

    1 or 2 gpus 8x dosent break it but as with anything you start throwing more at it, performance gains drop, that said 3 gpus will still scale in some titles and others that dont stutter is gone, and 7970s make more sense to an extent, but maybe the OP has a good deal on the 6900 series

    as of this point ive yet to find any review or tests showing full on scaling of more then 2 gpus with PCIe lane bandwidth

    show me Linus testing 3x gpus running x8 x8 x8 and x4x4x8 etc im willing to bet performance drop will be easy to spot.
     
  15. Yo_Wattup New Member

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    I think the question is why WOULD I buy a 7970? 1. they are over priced (here in Australia they are the SAME price as a 6990) and 2. I already have a 6950, so why not CFX it with the highest card I can, uber-futureproofing my setup? :)

    But (for example) an overclocked 7970 might have the same bandwidth usage as a 6990, are you saying you shouldn't crossfire 2x 7970s in 8x mode?
     
  16. f22a4bandit

    f22a4bandit

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    I believe Crazy is saying that if you take a dual GPU card like the 6990, you're effectively halving the lanes between the two onboard GPUs. If it was on an 8x slot, you'd be splitting it to 4x and 4x for each GPU. You wouldn't be doing that if you had two single GPUs on separate lanes.
     
  17. BrooksyX

    BrooksyX

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    Why not just pick up a used 6950 for cheap and crossfire them. Would make for a pretty sweet systems and I am sure you could find a deal on a nice 6950.
     
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  18. JrRacinFan

    JrRacinFan Served 5k and counting ...

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    Quite a paradox statement; you can always pair a 2nd 79xx card at a later date.
     
  19. Yo_Wattup New Member

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    I know but 8x still gives 8x worth of bandwidth right? :confused: Should we not be comparing the total bandwidth of the 6990 to the 8x's lane maximum throughput? (That took a long time to choose the right words :roll:). Say for example if an OCed 7970 had the same performance as a 6990, and they were both on 8x lanes, why would the 6990 be more bandwidth-limited? Considering that each of the 2 gpus on the 6990 would be half the power of the single one in the 7970

    Yeah but I gotta know when to stop, I can't just keep getting the latest and greatest all the time... I want to be money-wise and future-wise. :) And as I said before, prices here are the same for 6990 and 7970, and eventually if a 6950 + 6990 setup is not enough (hopefully that won't be for a while) I can sell the 6950 and get another 6990 (and possibly the necessary nuclear power plant to go with it :roll:) which overall will still be cheaper and perform better than any 7970/50 CFX setup. :D
     
  20. JrRacinFan

    JrRacinFan Served 5k and counting ...

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    No it won't. 79xx's would be easier on the electric bill and perform similar to 6990's. Also, quit trying to futureproof, there is no such thing. Also going 2x 6990's, odds are would need to get a new power supply, so you have added cost there.
     
    Last edited: Jan 29, 2012
  21. brandonwh64

    brandonwh64 Addicted to Bacon and StarCrunches!!!

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    a 7970 would benefit you the most. It will eliminate all crossfire issues and still give you just as good performance.
     
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  22. Super XP

    Super XP

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    Regardless of results, you want to at the very least guarantee maximum performance from a monster such as the HD 6990. So I would say yes PCIe x 16 is the way to go, though 8x will still do the job with a drop in overall performance.
     
  23. Yo_Wattup New Member

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    How is there no such thing as futureproofing? What about the people that got 2x 4870x2s when they came out? Thats a similar situation and those cards are still kickin ass. And I know how power supplies work, I'm gonna get a new power supply either way, I want something with removable cables like the AX Series, coz right now theres a bunch of unused cables in my case which is kind of annoying.

    Good point, but I still believe the positives of getting a 6990 outweigh the positives of getting a 7970. I shall do some more research on CFX issues though. :toast:
     
  24. JrRacinFan

    JrRacinFan Served 5k and counting ...

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    OK so you agree with Brandon but yet still attempt to try and flaw my thinking? I have a feelign you think i am attempting at a troll which I am not. Just trying to give some friendly advice. What I mean by futureproofing, you are never going to have the latest and greatest there will always be something new to come out within a years time. Just saying AT THIS POINT IN TIME, 79xx would provide a better upgrade standpoint than a 6990, now if you were to go a 2nd 6950 that's a totally different story. I would agree with your upgrade then.

    EDIT:

    Although irregardless, on topic, yes it would be best on a 16x link so the internal bridge chip can then do 8x to each gpu.
     
  25. crazyeyesreaper

    crazyeyesreaper Chief Broken Rig

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    facepalm

    its not just bandwidth for the damn gpu theres overhead on keeping all that shit in sync,

    ever see a bunch of people hold hands and try to run forward? what happens some people are faster then others some lag behind,

    it takes drivers and bandwidth both PCIE and System memory bandwidth to keep the cards in perfect Sync so they render frames properly

    its DRIVER OVERHEAD

    7970 + 7970 means driver only has to deal with 2x GPUs and both would be on x8 lanes meaning equal bandwidth

    6990 on x8 = x4 x4 for both gpus internally, the 6950 / 6970 or w.e for Trifire gets x8

    the driver dosent see 2 fucking graphics cards it sees, 3 GPUs and it now has to budget bandwidth and keep those cards in sync, the performance drop isnt going to be mind shattering terrible it will still scale, just not as well

    each gpu you add to a system increases graphics driver overhead requires greater and greater memory bandwidth to feed them and stresses PCIe bandwidth has it gets cut down for more cards

    to give you an idea

    a GTX 480 yes slower then a 6990 on average loses 7% performance when dropped from x16 to x4 link

    now 6970 is same performance as a GTX 480 but driver difference can affect this but for sake of making this easier to understand

    if a GTX 480 loses 7% performance going from x16 to x4
    X16 to x8 it loses 2% now if we look at the data it means performance drop from dual gpu on x16 slot is miniscule nothing to worry about, but when you put another GPU in you cut bandwidth to BOTH gpus on the 6990 effectively making them x4 x4

    now that would be a 7% drop in performance on a SINGLE CARD what happens when 2 GPUs hit that same wall 14% now look at the other GPU on x8 link thats 2% your now in theory at 16% performance hit, now

    say scaling is 80% on the 2nd GPU so your getting a good gain a 3rd GPU on average only scales another 30% or so unless pushing ultra high resolutions aka greater then 2560x1600

    now if we look at PCIE scaling and its potential performance draw back you might on Z68 at 1920x1200 only see a 10-15% improvement from a 3rd graphics card, in most cases the 4th gpu aka 2x 6990s pretty much gives 0% in a quad gpu set up unless forcing extreme res aka 6 screen eyefinity.
    [​IMG]
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