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Question about dual PCI-E Motherboards
    #15025726 -

Okay so I'm shopping for dual PCI-E Motherboards and I keep noticing these boards that say they have 2 PCI-E X16 slots, but then say (X16, X4), WTF?
So is it two X16 slots or one X16 slot and one X4 slot?
Here's some motherboards that are stating this.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138318
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131764

I wanna do a crossfire setup so I wouldn't want to be putting a X16 video card into a X4 slot, that would totally knock down the cards potential right?


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Re: Question about dual PCI-E Motherboards [Re: gref]
    #15027049 -

The board can accept two 16 lane PCIe devices, but only one will use 16 and the other 4.  Depending on what video card you're using it may bottleneck, but honestly PCIe 2.0 has a pretty high bandwidth at 16 lanes.  So PCIe 2.0 is roughly 500 MB/s per lane so 4x is 2 GiB/s.  If your card doesn't saturate that it should be ok. 

You can correlate cost to the number of PCIe 16x lanes in most cases.  If you're doing a large upgrade and buying top to mid level video cards then spend the extra $100 to get the dual 16x link motherboard.  While most motherboards with the latest technology accurately space dual 16x to fit the large size of top-end GPU's, some that only have one 16x link may not... idk.


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Science is a way of thinking much more than it is a body of knowledge. - My hero, who will be forever remembered, Carl Sagan.


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Re: Question about dual PCI-E Motherboards [Re: gref]
    #15027371 -

Here's some comparisons:

http://www.hardocp.com/article/2010/08/25/gtx_480_sli_pcie_bandwidth_perf_x16x16_vs_x4x4/
http://www.overclock.net/nvidia/819348-16x-16x-vs-16x-4x-gtx.html

Basically there's no MAJOR real-world difference between 16x/16x, 16x/8x, and 16x/4x. You would probably only notice the difference in benchmark scores.
Although depending on the board, running a card in the 2nd slot may drop the first slot to 8x.
It doesn't even really matter all that much, but I would try and go for a higher end board myself capable of at least 16x/8x.
But running 2nd card at 4x you are talking about MAYBE a 5% decrease in performance, if that.. so it's no big deal.


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Re: Question about dual PCI-E Motherboards [Re: Shroomism]
    #15029812 -

Thanks Synaptrip, very educational.

And thanks shroomism those links were very helpful.

Yeah I got an HD 5750 and I want some more power for BF3 so I'm gonna get another and crossfire it but I need a new motherboard, the x16 x4 ones are like 50$ which a relatively low price.
.....
After looking around though I decided to keep the same mobo and get a 6870, All factors included I'd be paying the same amount for the 6870 2gb as I would for the 5750 crossfire setup. Plus I get a copy of Deus Ex for free!
great deal.

http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.731484

Edited by gref (09/04/11 08:22 PM)

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Re: Question about dual PCI-E Motherboards [Re: Shroomism]
    #15030751 -

Hmm, the link I looked at didn't address the performance hit as a proportion of performance gain due to use of the second card, so I'm not so sure how much info that tells us.  Instead, the stats were given in terms of frames per second lane speed in dual card configuration only

If the x4 and x16 cards only loose 5% performance, that might not seem to bad, but what if the second card on a dual x16 system only added 10% frames per second over the single card's performance? This would mean you've lost 50% performance from a x4 lane. 


As a result, I would think the better metric would be to compare the difference in FPS in a x16 x16 and x16 x4 setup and then take the ratio of that difference with the improvement in FPS gained by using the second card.

Especially since the drivers and options of cards often change their output as a function of FPS, that 5% difference may represent a much greater loss in effective computation power.

I don't know, but that's what came to mind.  Those statistic might not be measuring what people think they are measuring.

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Re: Question about dual PCI-E Motherboards [Re: johnm214]
    #15030997 -

> As a result, I would think the better metric would be to compare the difference in FPS in a x16 x16 and x16 x4 setup

Even then, you have to be careful.  There are several ways that software can use multiple GPUs, and these each have pros and cons.  For example, alternate frame rendering (AFR) often suffers from micro-stuttering (a random 'lag' or stutter in the displayed video), but because performance is often averaged over a second (FPS), the micro-stutter is not seen by benchmarks.  A benchmark may show no difference in a x16/x4 setup compared to an x16/x16 setup, even though there is one, due to the averaging of performance over some time period.  Any strangulation caused by the lower bandwidth card will be averaged out by the benchmark, but can still result in a visual defect that is highly annoying.


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Just another spore in the wind.

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