Crossfire Question

[nicehat]

How exactly does it work. I know that the frames are split up between the two cards and that the master card outputs the signal as one image, but Im looking for more information. Why cant the other card add its memory to the overall video buffer? Whats the problem with that? 

 

I did a quick google search and I couldnt find what I was looking for. Any insight/articles would be appreciated. 

[Vitalius]

How exactly does it work. I know that the frames are split up between the two cards and that the master card outputs the signal as one image, but Im looking for more information. Why cant the other card add its memory to the overall video buffer? Whats the problem with that? 

 

I did a quick google search and I couldnt find what I was looking for. Any insight/articles would be appreciated. 

Because, effectively, both cards are working on the exact same frames. So they both have to have the exact same stuff in it's VRAM. Think of it like RAID 1, but with VRAM. 

The frames are split up between the two cards, but to stay in sync, they have to be working on them at the same speed (thus meaning they have the be the same clock/mem speed), and the same things (thus the RAID 1 of VRAM, effectively).

[nicehat]

Because, effectively, both cards are working on the exact same frames. So they both have to have the exact same stuff in it's VRAM. Think of it like RAID 1, but with VRAM. 

The frames are split up between the two cards, but to stay in sync, they have to be working on them at the same speed (thus meaning they have the be the same clock/mem speed), and the same things (thus the RAID 1 of VRAM, effectively).

 

I see. Another couple of questions, why is a xfire bridge necessary? Why cant the PCIe lanes provide the needed bandwidth ala Volcanic Island GPUs? How do PCIe 2.0 vs 3.0 xfire setups compare?

[Guest]

pci 2.0 x 16 is the same bandwidth as PCI 3.0 x 8

[needmorewood]

the new cards dont need it aparently

[nicehat]

the new cards dont need it aparently

 

yep, thats my question. If the bandwidth of PCIe 3.0 hasnt changed, but the existence of the bridge has, then why do we need bridges/ribbons for the 7000 series of cards? This has been on my mind especially since the 280X is a rebrand of the 7970, yet does not need the bridge.  

[Virgule]

Because, effectively, both cards are working on the exact same frames. So they both have to have the exact same stuff in it's VRAM. Think of it like RAID 1, but with VRAM. 

The frames are split up between the two cards, but to stay in sync, they have to be working on them at the same speed (thus meaning they have the be the same clock/mem speed), and the same things (thus the RAID 1 of VRAM, effectively).

So what's the added value of having 2 if both have to do the same calculations and rendering ?

[Freaky_spider]

yep, thats my question. If the bandwidth of PCIe 3.0 hasnt changed, but the existence of the bridge has, then why do we need bridges/ribbons for the 7000 series of cards? This has been on my mind especially since the 280X is a rebrand of the 7970, yet does not need the bridge.  

well it used to be necesairy and they took it for granted to use a CF/SLI bridge, but AMD did some testing with PCIe 3.0 and saw that there wasnt any difference

[Vitalius]

I see. Another couple of questions, why is a xfire bridge necessary? Why cant the PCIe lanes provide the needed bandwidth ala Volcanic Island GPUs? How do PCIe 2.0 vs 3.0 xfire setups compare?

For your last question, there is little to no difference. Here is a makeshift chart to show you approximate bandwidth comparisons:

PCI-e 3.0 x16

PCI-e 2.0 x16 = PCI-e 3.0 x8

PCI-e 1.0 x16 = PCI-e 2.0 x8 = PCI-e 3.0 x4

PCI-e 1.0 x8 = PC-e 2.0 x4 = PCI-e 3.0 x2 (which technically doesn't exist, it's either x1, x4, x8, or x16 though x1 is rare IIRC).

So PCI-e 2.0 x16 has the same bandwidth as PCI-e 3.0 x8. Each new PCI-e revision is a doubling of bandwidth. 

Because of this, using PCI-e 2.0 x16 over PCI-e 3.0 x8 makes negligible difference (within margin of error). 

I'm not sure on your first two questions. Let me research it a bit unless someone else knows.

Possibly this:

 

well it used to be necesairy and they took it for granted to use a CF/SLI bridge, but AMD did some testing with PCIe 3.0 and saw that there wasnt any difference

 

So what's the added value of having 2 if both have to do the same calculations and rendering ?

... I think I didn't word what I said correctly.

They have to know what the other card is doing so they stay in the rhythm, or runt frames occur. So every GPU has to have a complete copy of what ALL the GPUs are working on, or they won't know what frame they are working on, or when to display it.

However, each GPU renders a different frame. Thus allowing their speed to increase because they are working on multiple frames in parallel, it's basically a larger version of a multi-threaded program.

[helping]

So what's the added value of having 2 if both have to do the same calculations and rendering ?

I believe multi GPU can work in one of two ways.

 

disclaimer: I am speculating on how this likely works, this is nothing but me guessing. 

 

One GPU renders one portion of a screen, meaning one card renders half the screen, the other card renders the other half. 

 

Or they alternate frames so that each card is doing about half the work. In a sequence of four frames it would like like GPU1 > GPU2 > GPU1 > GPU2

 

They will require having identical VRAM because it still needs to determine what to render, and will need everything necessary to render what the other card could possibly render. 

 

If they kept different objects in memory, if one card needed to render something the other card has loaded into VRAM, it would either need to seek it from the other cards VRAM, introducing latency and increasing bandwidth demand, or load it onto it's own VRAM. 

[Virgule]

I believe multi GPU can work in one of two ways.

 

disclaimer: I am speculating on how this likely works, this is nothing but me guessing. 

 

One GPU renders one portion of a screen, meaning one card renders half the screen, the other card renders the other half. 

 

Or they alternate frames so that each card is doing about half the work. In a sequence of four frames it would like like GPU1 > GPU2 > GPU1 > GPU2

 

So core clock should be able to almost double when on crossfire ?

[Vitalius]

So core clock should be able to almost double when on crossfire ?

Effectively, yes. The GPUs are running at the same speeds they usually do, it's just that there are now 2 of them working parallel. 

This is the equivalent of a single-core CPU at 3.4GHz becoming a dual-core CPU at 3.4GHz, but with both cores working on the same program.. It's the same concept. 

[nicehat]

So core clock should be able to almost double when on crossfire ?

 

I thought you maintain the same clock, but since the compute is split up b/w both GPUs you effectively get twice the frames with the same clock. Or I guess you could look at it as a single card with double the clock. I think thats what you mean? 

If this is the case, why doesnt xfire get exactly double the frames as a single  card? Why does scaling decrease with more cards? Is it software or hardware related?

[sperkowsky]

it splits up the game evenly giving the graphics cards a little extra wiggle room but honestly for how much it costs sometimes isnt worth it

[Virgule]

I'm quite dissapointed, I just bought my 2nd sapphire vapor-x 7970 and will receive it today. Maybe I should sell them both and get an r9 290x.

[nicehat]

it splits up the game evenly giving the graphics cards a little extra wiggle room but honestly for how much it costs sometimes isnt worth it

 

you can get a 7950 for next to dirt cheap. Why not eek out the added benefits if it means another $180 or so? And all the free games? 

[sperkowsky]

you can get a 7950 for next to dirt cheap. Why not eek out the added benefits if it means another $180 or so? And all the free games? 

I dont see 180 dollars as dirt cheap, and when you think about ti you sell one you have 400 dollars and you can pick up a 7970 with 100 dollars to spare and better performance. Unless you get a deal on ebay or something. and the never settle bundle kinda sucks righht now

[nicehat]

I dont see 180 dollars as dirt cheap, and when you think about ti you sell one you have 400 dollars and you can pick up a 7970 with 100 dollars to spare and better performance. Unless you get a deal on ebay or something. and the never settle bundle kinda sucks righht now

 

The 7950 is essentially the same thing as a 7970. The 7970 tops the 7950 by about 5% (about 5 frames max in the real world according to most benchmarks). Overclock the vastly cheaper 7950, and that gap doesnt exist.

 

http://hexus.net/tech/reviews/graphics/34761-amd-hd-7950-vs-hd-7970-clocks/

 

And I dont know what prices are like where you are from, but no one would buy a 7950 for $400

[Vitalius]

If this is the case, why doesnt xfire get exactly double the frames as a single  card? Why does scaling decrease with more cards? Is it software or hardware related?

 

Inefficiencies, latency, bad drivers, etc. Basically, lots of things add up to the point that it isn't 100% efficient. That's why you usually see 60-80% FPS increase for adding a single GPU. 

So it's related to both Software (drivers) & Hardware (latency). Not only that, but remember that Crossfire pushes the CPU more too. And CPU's can be bottlenecks as well.

it splits up the game evenly giving the graphics cards a little extra wiggle room but honestly for how much it costs sometimes isnt worth it

 

This basically. Sometimes it isn't worth it. Sometimes it is. It is worth it with cheaper cards (like the 7XXX series after price cuts) because you aren't going to be getting that kind of performance for that price. 2 7950's are $360 I think, and that would roughly give you the performance of a 290X. Obviously $360>$550.

I'm quite dissapointed, I just bought my 2nd sapphire vapor-x 7970 and will receive it today. Maybe I should sell them both and get an r9 290x.

Why are you disappointed?

[helping]

I dont see 180 dollars as dirt cheap, and when you think about ti you sell one you have 400 dollars and you can pick up a 7970 with 100 dollars to spare and better performance. Unless you get a deal on ebay or something. and the never settle bundle kinda sucks righht now

Can you try to clarify because what you've said makes practically no sense.

 

I'm quite dissapointed, I just bought my 2nd sapphire vapor-x 7970 and will receive it today. Maybe I should sell them both and get an r9 290x.

No, you're good. Driver support has come a long way and most of the dual GPU issues have been ironed out. Depending on the game, some titles have near linear crossfire scaling. It doesn't scale linearly because of the nature of what is happening, it is limited by technology (technological infrastructure) and compute required to have functioning drivers, frame pacing, and the optimizations and how well a specific game supports crossfire.

Games that don't support crossfire or any multi GPU setup well or at all rarely ever need the power that a dual GPU setup would provide anyway, meaning one card will be sufficient for games that exclude multi GPU technologies.

You don't typically go above 2 card multi GPU setup, because the improvements after that are typically pretty awful. Two 7970's in crossfire in terms of raw horsepower will destroy or surpass any single current GPU solution.

[Virgule]

Can you try to clarify because what you've said makes practically no sense.

 

No, you're good. Driver support has come a long way and most of the dual GPU issues have been ironed out. Depending on the game, some titles have near linear crossfire scaling. It doesn't scale linearly because of the nature of what is happening, it is limited by technology (technological infrastructure) and compute required to have functioning drivers, frame pacing, and the optimizations and how well a specific game supports crossfire.

Games that don't support crossfire or any multi GPU setup well or at all rarely ever need the power that a dual GPU setup would provide anyway, meaning one card will be sufficient for games that exclude multi GPU technologies.

You don't typically go above 2 card multi GPU setup, because the improvements after that are typically pretty awful. Two 7970's in crossfire in terms of raw horsepower will destroy or surpass any single current GPU solution.

Thanks for the cheer up.