SLI questions

[aandril]

1) x2 in SLI doesn't double the performance, but what can I count on performance wise as a general rule? 125% performance, 150% perf? 175?...

 

2) Silly question, but Are SLI cards put into their own PCIs? Say I've got one in a PCI 16x, and another in an 8x PCI?

 

3) I may or may not have read somewhere that if there are two cards, one in a x16, and one in a 8x, they will be downgraded to 8x + 8x?

Eh, why is there only one 16x PCI slot on a mobo if that particular mobo is MADE to do SLI?

[r2724r16]

2 minutes ago, aandril said:

1) x2 in SLI doesn't double the performance, but what can I count on performance wise as a general rule? 125% performance, 150% perf? 175?...

Depends on the specific game.

2 minutes ago, aandril said:

2) Silly question, but Are SLI cards put into their own PCIs? Say I've got one in a PCI 16x, and another in an 8x PCI?

If you're putting two GTX 970s in SLI (for example), they each go in their own slot. 

2 minutes ago, aandril said:

3) I may or may not have read somewhere that if there are two cards, one in a x16, and one in a 8x, they will be downgraded to 8x + 8x?

Depends on how many PCIe lanes your CPU can provide. 

2 minutes ago, aandril said:

Eh, why is there only one 16x PCI slot on a MOBO if they're MADE to do SLI?

Which specific motherboard are you talking about? 

[aandril]

1 minute ago, r2724r16 said:

Depends on the specific game

If you're putting two GTX 970s in SLI (for example), they each go in their own slot. 

Depends on how many PCIe lanes your CPU can provide. 

That's why I said as a general rule...?

Okay.

What do you mean by how many the CPU can provide?

[Phentos]

SLI scaling is entirely dependent on SLI profiles created by Nvidia in a driver release, along with how the game itself handles it. SLI support is extremely spotty nowadays, so I wouldn't expect any better than maybe 150% scaling across the board. And that's an optimistic guess.

 

SLI requires each GPU to be installed in both full length PCIe slots on your motherboard. They will be configured for x8 if you have a non-HEDT (X99/X299/X399) CPU and motherboard.

 

For many gaming scenarios there isn't a substantial difference in performance between x8 PCIe lanes and x16 for multi-GPU setups. If you are gaming in 4K or above with very demanding games, then the differences will become much more apparent. 

[r2724r16]

1 minute ago, aandril said:

That's why I said as a general rule...?

Okay.

What do you mean by how many the CPU can provide?

There's no general rule. SLI can double your performance in some games, while it can even hurt performance in others. 

 

And I meant your CPU can only provide a certain number of PCIe lanes. 

[TheGlenlivet]

X8 vs X16 doesn't degrade performance significantly.  Don't worry about that if you do SLI with a Z170/270/370/390 or AM4.

If you haven't bought anything yet and are on a budget, you should buy the best single card you can afford.

1 great card is better than 2 good cards.

[Phentos]

5 minutes ago, aandril said:

That's why I said as a general rule...?

Okay.

What do you mean by how many the CPU can provide?

A CPU has a specific number of PCIe lanes it can directly address with its IOMMU and PCIe I/O circuitry. Most mainstream CPUs can typically directly serve 16 or 20 PCIe lanes. HEDT CPUs like Intel's i7/i9 X series CPUs or AMD's Threadripper lineup can address many more PCIe lanes directly, like 28-44 for Intel and 60 for Threadripper.

[r2724r16]

Just now, Phentos said:

60 for Threadripper

Isn't it 64? Or am I mistaken 

[Phentos]

3 minutes ago, r2724r16 said:

Isn't it 64? Or am I mistaken 

4 are reserved for the chipset.

[Nexxus]

The first point is why you should avoid SLI, and I say this as someone who had SLI 680s.

The performance is very inconsistent, the reason you are not being given a general rule is that there isn't one, the best results I saw here BF where I would get about 80%ish bump over 1 680, with that said. the BF game increase in performance is a rarity. You will generally be looking at considerably less of an increase than that. 

In some cases, you will literally see a 0% increase and while rare in some cases you will actually get worse performance than if you just ran a single card.

Based on this, just don't do it imo, if you want more power sell what you have and get something better. I will continue to say SLI makes no sense unless you have the near enough or the best card on the market already and it just isnt good enough. That or you get that second card disgustingly cheap.