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post #11 of 17
Technically it does, because each card is still communicating with the host processor and host memory via it's own dedicated PCIe slot. With that statement we will make the assumption that the graphics cards are installed in a configuration where each card is communicating to the host via dedicated x16 or x8 PCIe lanes (and not some "bandwidth" sharing arrangement).

But I certainly wouldn't be making any claims that just because the memory interface has doubled in size that there are linear gains in performance. The video card manufacturers have already established that a larger interface doesn't necessarily translate into better performance.

Take into consideration the numerous nVidia high performance graphics cards that run on a 256-bit memory interface.
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post #12 of 17
If you were to take a 7870 XT and a 7950 and clock them so that they deliver the same raw performance, then you'll see the 7950 edge out over the XT due to the bus. However, that's within the same architecture. The 680 and 7970 are/were neck-and-neck performance-wise, and the 780Ti is a bit more powerful than a 290X which is a bit more powerful than a 780, all this despite Nvidia's smaller bus in each case. I believe they compensate with a significantly faster memory clock, but I'm not really familiar with their stuff. AMD was the cheapest 2GB option so I went with them.
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post #13 of 17
Quote:
Originally Posted by wevsspot View Post

Technically it does, because each card is still communicating with the host processor and host memory via it's own dedicated PCIe slot. With that statement we will make the assumption that the graphics cards are installed in a configuration where each card is communicating to the host via dedicated x16 or x8 PCIe lanes (and not some "bandwidth" sharing arrangement).

But I certainly wouldn't be making any claims that just because the memory interface has doubled in size that there are linear gains in performance. The video card manufacturers have already established that a larger interface doesn't necessarily translate into better performance.

Take into consideration the numerous nVidia high performance graphics cards that run on a 256-bit memory interface.

Understood, I'm just trying to see if you CrossFire two 256 bit bus cards like the 7870, would they have the same bandwidth as an equivalent 512bit bus card e.g. an R9 290 (i.e. on the memory side only, not factoring the GPU)
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post #14 of 17
The available pipe would be the same but the memory bandwidth will be different because of the difference in the effective memory clock of the two cards you are comparing.

The R9 290 has 5000Mhz effective memory clock

The 7870 Ghz Edition has 4800Mhz effective memory clock
Edited by wevsspot - 11/15/13 at 6:07am
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post #15 of 17
So once the memory is clocked to the same speed then they'd be equal
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post #16 of 17
Aren't the 290s 512-bit? That's 33% more bandwidth than a 7900, even disregarding the clocks.
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post #17 of 17

Yes the R9 290 has a 512-bit memory interface, but the OP is comparing a single R9 290 versus Crossfired 7x series and wondering if the memory "pipe" could be equal to or greater than the 512-bit of a R9 290 card.

But lets go back to how the memory bus works on a video card. Communication between the gpu core and the vRAM occur over the memory bus. The gpu core can communicate as much information to the vRAM (and visa-versa) as the memory bus capacity will allow. Think lanes of traffic.................

But if there aren't enough memory chips (capacity) to saturate the memory bus, then it doesn't matter whether you have a 256-bit interface, 384-bit interface or even a 512-bit interface. The other part of the equation is how efficient the card manufacturer is at directing the traffic.

Take the nVidia GTX Titan for example; 6Gb of vRAM on a 384-bit memory interface

Now look at the flagship AMD offering R9 290X; 4Gb of vRAM over a 512-bit interface

Apparently the nVidia design engineers believe that the 384-bit interface on the Titan is more than enough for addressing all 6Gb of vRAM if required.

Now in real world for the crossfired AMD 7x series. The bandwidth of the memory interface is only as important as the number of memory chips (capacity) that the video card needs at any given time based on whatever task is on hand. And we won't even get started on buffering.

Frankly, I think we are paying a lot of attention to a "number" that really isn't that important to the overall performance of the video card(s) assuming that the manufacturer's brain trust knows what they are doing.

And all evidence on the table suggests that those groups at both AMD and nVidia are doing an outstanding job from cradle to grave. Just look at the performance numbers of today's cards - and icing on the cake is that in many cases both companies have been able to offer incredible performance gains while maintaining (and in many cases) even reducing the cost of manufacturing new generation cards versus the previous generation of cards.

That means more performance for us and sometimes even at better pricing.
Edited by wevsspot - 11/15/13 at 6:33am
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