Why is the 4 GFX cards the "limit" in SLI/Crossfire

[EcoBoost]

There aren't enough PCIe slots for 5-way with dual-slot, and why would you want to run 5+ single-slot GPUs

[Jarsky]

Bandwidth is fine, depending on the motherboard and if the Gpu is liquid cooled and i/o is single slot; PCI-E slots are a problem either. The problem is scaling, after two GPUs scaling is horrible.

 

That depends on the board/cpu

The reason scaling is horrible is you may have something like a Z87/Z97 with say a 4790K then thats 28 PCI-E Lanes.

You connect 3 cards, thats x8/x8/x8. You connect 4 cards, thats x8/x8/x4/x4 - so the available bandwidth drops down on both cards 3 & 4.

There is negligible difference for graphics cards between x16 and x8, but there is a substantial drop from x8 to x4.

 

You end up with, depending on games something that ends up about

1-way 100%

2-way 180%

3-way 250%

4-way 270%

 

Compared to an X99 with 40 lanes at x8/x8/x8/x8 which might look more like

1-way 100%

2-way 180%

3-way 250%

4-way 320%

 

 

Earlier boards prior to Z77+PLX were even worse for scaling because they only had 16 PCI-E lanes. So if you had 4 cards they would drop to 4x/4x/4x/4x.

It's nothing to do with the "their arent 5 slots". There are plenty of boards with 6+ PCi-E slots.

The problem is bandwidth.

 

Disclaimer: I made the above figures up, but you get the idea.

[asim1999]

Software limitations/PCIe bus limitations and such. I've seen a rig run 8 GPU's with four 7990's before, but it probably wasn't used for gaming. Only rendering and mining machines would benefit from over 4 GPU's. I think NVIDIA's current SLI limit is five GPU's, but I may be wrong.

Can u actually run 4 physical 7990s? (4 dual GPU cards)?

[Jarsky]

Can u actually run 4 physical 7990s? (4 dual GPU cards)?

 

Yup, you can run them as individual displays, or utilise the CUDA cores for GPGPU processing.

But you can only Crossfire 2 of them.

[Godlygamer23]

Yup, you can run them as individual displays, or utilise the CUDA cores for GPGPU processing.

But you can only Crossfire 2 of them.

AMD doesn't use CUDA cores. They use Stream Processors.

[Dabasepc]

technologies isnt very advanced.

[Jarsky]

AMD doesn't use CUDA cores. They use Stream Processors.

 

 

Same thing. Just using it as a generic term.

[asim1999]

Yup, you can run them as individual displays, or utilise the CUDA cores for GPGPU processing.

But you can only Crossfire 2 of them.

Oh so u mean i can use 2 for gaming and 2 for rendering/

It would be cool if u could run 4 dual cards and get good scaling 

[Godlygamer23]

Oh so u mean i can use 2 for gaming and 2 for rendering/

It would be cool if u could run 4 dual cards and get good scaling 

Your power draw would be very high.

[asim1999]

Your power draw would be very high.

Yeah, IKR. If u can afford 4 7990s, then a 900D and dual AX1500is wont be out of the question either 

[juretrn]

Best guess is driver support.

[Jarsky]

Oh so u mean i can use 2 for gaming and 2 for rendering/

It would be cool if u could run 4 dual cards and get good scaling 

 

For sure.

You can have 2 cards in SLI/Crossfire and a 3rd that you just dedicate to PhysX.

You'll need a pretty high end motherboard though to get benefit out of 2 x 7990's vs 3 x 7970's. It's not really practical.

 

Of course for gaming, you'd be better off just getting a single GTX 970 these days (7990's potentially arent the best example lol)

[Godlygamer23]

For sure.

You can have 2 cards in SLI/Crossfire and a 3rd that you just dedicate to PhysX.

You'll need a pretty high end motherboard though to get benefit out of 2 x 7990's vs 3 x 7970's. It's not really practical.

 

Of course for gaming, you'd be better off just getting a single GTX 970 these days (7990's potentially arent the best example lol)

Something that should be mentioned is that PhysX does not work on AMD cards. That is, the GPU will not do anything.

[TheKDub]

To add to this, Probably a limit of PCI lanes, and at 4 graphics cards, you'll start getting close to the limit of a standard 15 amp circuit in your house can deliver power wise,

 

 

That is unless they are quite power efficient and don't use much amperage.

[Cheddle]

Why is there a limit of a maximum of 4 graphics cards that you can put in SLI/Crossfire? In theory why not just get a mobo that has a 5th PCIe slot (none exist, but if they did), and slap another one in there.

 

Is there a reason four is the max? I did some youtubing but nothing came up?

 

Why not get a case the size of my floor to the roof, a 12 foot motherboard, and get 50way SLI? 

I know that example in silly, but why stop at 4? UNLESS

 

a triangle has 3 sides, 1 eye, 3+1 =4, 4way SLI, illuminati confirmed

 

Really though, who made this magical maximum of "4 gpus"

 

Current I/O is not designed with more than 4 GPU's intended.

 

the problem is each GPU talking to each GPU and the CPU. currently they do this mainly via the CPU and sometimes the south-bridge. The Southbridge is bad at this as its not designed for such high thruput.

 

If you redesigned PCIe with 50 GPU's in mind then sure, you could have 50 GPU's each rendering a small section of the screen.

 

Personally if this was the approach and I was designing it I would have separate "GPU" cards and "RAM" cards. with multiple GPU's sharing a RAM card, imagine having 2,000 CUDA cores per card all forming one ginormous network of CUDA cores that can communicate directly with each other and can access the same block of ram using the same architecture as a single card.  latency would need to be non-existent.

 

one card across many cards.

[marat569]

I just watched linus's video on the 30 lane crap; but what if we had a new 50 lane CPU, and AFAIK: 8x 8x 8x 8x 1x > 8x 8x 8x 8x 0x

 

1x better than nox

 

Thank you guys for the awesome convo though!