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[AMD] Modernizing multi-GPU gaming with XDMA

post #1 of 40
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MODERN MULTI-GPU WITH XDMA

Picture7.pngAt a principle level, XDMA dispenses with the external bridge by opening a direct channel of communication between the multiple GPUs in a system. This channel operates over the very same PCI Express® bus in which your AMD Radeon graphics cards are currently installed. The exclusive function of that bus is to shuttle graphics data between GPUs and your processor, so it’s already well suited to the new task of collecting and showing the data each GPU is working on when playing games.

It just so happens that the PCI Express bus also provides a tremendous amount of bandwidth—far more than can be allocated to today’s external bridges! As noted by Anandtech in their comprehensive analysis of XDMA, the bandwidth of an external bridge is just 900MB/s, whereas PCI Express® can provide up to 32GB/s with a PCIe 3.0 x16 slot (about 35x more bandwidth).

In dynamically taking a portion of that superhighway to negotiate rendering with multiple GPUs, AMD CrossFire can efficiently negotiate UltraHD scenarios. This is one of the many reasons why we say that the AMD Radeon R9 290 and R9 290X are uniquely suited, at a hardware level, for gaming at 3840x2160.

Diving more deeply into the technology, XDMA specifically and directly connects the “display controllers” on the respective GPUs in an AMD CrossFire configuration. These display controllers are responsible for taking a rendered scene in a game from the GPU pipeline and formatting it to send over the display cable to a monitor. XDMA provides an easier and more extensible method of transferring the frame from the GPU it was rendered on, to the GPU driving the display cable, using the high bandwidth of PCIe, while avoiding extra connectors and cables.
AMD has released this large article identifying their bridgeless crossfire system that was released in the 290 series of their graphics cards
All of this has been available knowledge for sometime but this deeper yet simple description is still news worthy
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post #2 of 40
you would think Nvidia isnt just sitting around and is also working on bridgeless tech. im glad we are finally putting all this unused PCIe bandwidth to good use.
post #3 of 40
Nice little article, thanks for posting.
post #4 of 40
I didn't actually know that the bridgeless technology had performance gains too. Great read, thanks.
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post #5 of 40
Well its not like thees are the first GPU's to be able to run in crossfire without an bridge. The 6750's and 7750's can do it for example. What I wan't to know is if this is actually something new or just rebranded old tech. The bridges where developed for a reason namely to combat insufficient bandwidth between cards. Is this actually an improvement or merely a cost cutting measure?

How well will it work on slower PCI-E slots like 3.0 x8, 2.0 x16 and 2.0 x8?

We need benches on this.
    
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post #6 of 40
Quote:
Originally Posted by Namwons View Post

you would think Nvidia isnt just sitting around and is also working on bridgeless tech. im glad we are finally putting all this unused PCIe bandwidth to good use.
I know that Nvidia has had bridgeless multi-gpu solutions since my SLI 9600GT days.
post #7 of 40
Why should the bandwidth of a bridge be limited to 900MB/s? It's a connector used solely and controlled by Nvidia or AMD. They can up the bandwidth whenever they want.
I'm all for more performance but I do like the look of nicely done bridges.
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post #8 of 40
Quote:
Originally Posted by AlDyer View Post

I didn't actually know that the bridgeless technology had performance gains too. Great read, thanks.

well when you think about it, it makes perfect sense. if the bridge does 900MB/s, then at 60FPS, that's 900/60=15MB/frame. that's not much to play with. 32000/60=500MB/frame. i'm stretching it a little (it's not dedicated), but that's quite a difference.

as for working with slower/fewer links, I can't see why not.
 
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post #9 of 40
Quote:
Originally Posted by Legonut View Post

Why should the bandwidth of a bridge be limited to 900MB/s? It's a connector used solely and controlled by Nvidia or AMD. They can up the bandwidth whenever they want.
I'm all for more performance but I do like the look of nicely done bridges.


Because the same reason you need higher quality ethernet cables for gbit over 100m, The quality of the connectors probably limit the amount of data that can be sent, Ever seen how easily those stupid little flexi bridges get damaged? Yeah..
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post #10 of 40
Quote:
Originally Posted by Legonut View Post

Why should the bandwidth of a bridge be limited to 900MB/s? It's a connector used solely and controlled by Nvidia or AMD. They can up the bandwidth whenever they want.
I'm all for more performance but I do like the look of nicely done bridges.

it doesn't work that way. bandwidth is limited by more than someone picking a number out of a hat. look at ATA 33/66/100/133. the end connector was the same, but the protocols changed, the hardware changed, and the number of pins in the cable doubled - not to mention the engineering that went into it to pull up those speeds. 900MB/s is 7.2Gb/s - that's faster than SATA 6Gb/s, and look at how long that took to achieve.
 
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