Whats best for sli PCI-E x16 both or x16/x8

gin23 · Mar 7, 2014

I see motherboards with two PCI-E x16 and PCI-E x16 and the second slot is x8.
Should I buy the one with two PCI-E X16 slots? Does it affect performance when doing sli?

Nikola Aslanov-1338074 · Mar 7, 2014

Best is 2 x16.
There will be very little difference with x16 +x8, but if you are planning for SLI/xfire, you are aiming for very high performance, so I'd say that every percent counts.
And the motherboards with dual x16 are not that expensive.

enemy1g · Mar 7, 2014

You are correct that two x16 lanes will give the best performance. However, all multi GPU will only ever have the need for dual x8 lanes, for now. The performance difference between two x16 lanes and two x8 lanes is very minimal, at best. Something like 1600 CL9 RAM vs 1866 CL9 RAM.

And motherboards with dual x16 lanes are actually quite expensive, and are usually associated with Extreme series CPUs (Two x8 lanes on an AMD board is quite rare, so talking about Intel here), so along with getting an expensive extreme series CPU, you'll need that expensive extreme series motherboard.

Most boards will support dual x8 lanes. A x16/x8 board is.. nonexistent? Because I believe if you're using SLI, it'll be using dual x8 lanes.

Nikola Aslanov-1338074 · Mar 7, 2014

That is right. The real world question is: x16 + x4 or Dual x8.
And the real world answer is - it does not matter. The performance drop is minimal.

gin23 · Mar 7, 2014

enemy1g :

So I cant get a mobo that is x16 + x8 if Im going to sli. The slots have to match?
If so Ill go with the x16 x2

cst1992 · Mar 7, 2014

You can if you can find one, but the x16 drops to x8 in SLI anyway.

enemy1g · Mar 7, 2014

The performance difference between a motherboard running at x16/x4 is noticeable. That is a fact. A motherboard running at x8/x8 is considered optimal without breaking the bank for a minimal performance increase.

Motherboards will not run in x16/x8, it's either going to run in x16/x4 (these boards are usually for single GPU setups), or x8/x8 (multi GPU setups).

Eggz · Mar 7, 2014

No one can answer this question correctly without more information. We need to know which chipset your considering because that will tell us the number of PCI-e lanes it supports. Each speed multiple takes up a lane. So, a PCI-e 16x slot can use up to 16 lanes, and 8x slot uses 8 lanes, etc.

For instance, the Intel Z87 chipset for the 4770k supports 16 PCI-e lanes. That means you can run SLI/Crossfire in an 8x8 configuration max. It doesn't matter what your PCI-e slots actually are. The result is that, even though you have 4 slots that are PCI-e v3.0x16, your expansion will not go beyond two video cards running in SLI or Crossfire at 8x each. It's just a limitation on the processor's chipset.

Another more important thing to consider is whether your processor can deliver information to the cards fast enough. That will likely be a bottleneck before the bandwidth of PCI-e, which has headroom to spare. So, pairing even an Intel i5-4670k with two gtx 780ti cards would likely result in a CPU bottleneck in certain use scenarios because of the processor being unable to feed both 8x PCI-e slots quickly enough. Just something to keep in mind.

Expansion turns out to be one of the primary reasons people buy into the Extreme platforms (e.g. X79 on the LGA2011 socket). For instance, the 3930k, 3960x, and 3970x, as well as this year's 4930k and 4960x, all support 40 PCI-e lanes. That gives you two GPUs running at 16x with 8 extra lanes left over. It's also part of the reason why SLI doesn't scale well past 2 cards. Another reason for poor scaling is that the SLI bridge's bandwidth is limited only to 1 GB/s, which is nothing compared to PCI-e v 3.0.

But I digress. Point is, the number of PCI-e lanes is an important consideration when you want multiple video cards or lots of expansion generally.

Mortis Angelus · Mar 8, 2015

Eggz :

No one can answer this question correctly without more information. We need to know which chipset your considering because that will tell us the number of PCI-e lanes it supports. Each speed multiple takes up a lane. So, a PCI-e 16x slot can use up to 16 lanes, and 8x slot uses 8 lanes, etc.

For instance, the Intel Z87 chipset for the 4770k supports 16 PCI-e lanes. That means you can run SLI/Crossfire in an 8x8 configuration max. It doesn't matter what your PCI-e slots actually are. The result is that, even though you have 4 slots that are PCI-e v3.0x16, your expansion will not go beyond two video cards running in SLI or Crossfire at 8x each. It's just a limitation on the processor's chipset.

Another more important thing to consider is whether your processor can deliver information to the cards fast enough. That will likely be a bottleneck before the bandwidth of PCI-e, which has headroom to spare. So, pairing even an Intel i5-4670k with two gtx 780ti cards would likely result in a CPU bottleneck in certain use scenarios because of the processor being unable to feed both 8x PCI-e slots quickly enough. Just something to keep in mind.

Expansion turns out to be one of the primary reasons people buy into the Extreme platforms (e.g. X79 on the LGA2011 socket). For instance, the 3930k, 3960x, and 3970x, as well as this year's 4930k and 4960x, all support 40 PCI-e lanes. That gives you two GPUs running at 16x with 8 extra lanes left over. It's also part of the reason why SLI doesn't scale well past 2 cards. Another reason for poor scaling is that the SLI bridge's bandwidth is limited only to 1 GB/s, which is nothing compared to PCI-e v 3.0.

But I digress. Point is, the number of PCI-e lanes is an important consideration when you want multiple video cards or lots of expansion generally.

Thanks for a great explanation.

But if I SLI/Crossfire two GPUs (x8+x8) with a 4790K (max 16 lanes) what happens if I also have a small WLAN-adapter (like this one: http://www.jimms.fi/tuote/TL-WN881ND) or sound card using those small PCI-E ports? Will that take 1 lane then and make the system even more inefficient? Or will the effect be almost unnoticeable?

P.s. Semi-sorry for nudging an old thread, but this is a valid question here I think.

cst1992 · Mar 8, 2015

Find an alternative, as usually the x1 slot shares bandwidth with the bottom x16 slot(you'll have to check with your motherboard if this is the case).
If it is, then your card in that slot will be very slow, as will other cards in SLI, as they use the same number of lanes(you can't have a x12/x4 solution; it's either x4 or x8 or x16 for all cards). I've read some ASUS boards have a PCI-E SSD slot which shares bandwidth with the bottom x16 slot, causing it to run at just x2 bandwidth.
I could be mistaken - if you think so, let me know.

Mortis Angelus · Mar 8, 2015

cst1992 :

I'm considering the Asus Z97 Pro Gamer as my next mobo. (http://www.asus.com/Motherboards/Z97PRO_GAMER/) together with 4790K. But I also want a WLAN adapter. I am, however, not sure if I ever will SLI or just upgrade GPU as I go along. For the latter I guess it won't be a problem, but otherwise?..

chenw · Mar 8, 2015

I just checked the specs on it, it looks like the bottom PCI-E lane is a chipset controlled lane, not CPU controlled lane, which means you are freely able to install a PCI-E x1 card in that without impacting your SLI.

Just for future reference, whether or not that bottom PCI-E lane is CPU or chipset controlled is EXTREMELY important for SLI on a Z97 motherboard, because one thing some may not realise is that if that bottom lane is CPU controlled, installing a card in that slot will cause your second PCI-E lane to drop to X4, and when that happens, you won't be able to activate SLI, period. SLI requires ALL cards involved in the SLI to be running at X8 minimum, although XFire have no such limitation. My motherboard's bottom PCI-E lane (Z97-UD5H) for example is CPU controlled, so I cannot actually use that bottom lane at all. From the specs on the manual, it states that the bottom X4 lane also shares lanes with the other 2 PCI-E 2.0 x1 slots, so it looks like it's Chipset controlled, so inserting a card here should not impact your SLI.

AFAIK Chipset lanes and CPU lanes are completely seperate, hence installing cards in Chipset controlled lanes should have no impact on the performance of CPU lanes, though not that it matters much, as GPU's often cannot use anywhere near the bandwidth X8 provides.

However, my opinion with SLI is that if you consider it, you are much better off SLI'ing from the very start, or I wouldn't recommend SLI'ing at all unless your card is double VRAM version or Titan, because one of the limitations of SLI/XFire is that it cannot solve VRAM bottleneck, so it would force you to upgrade your card you were going to SLI in the first place. With an SLI setup straight off the bat, at least you will get the performance of SLI setup from the very beginning.

cst1992 · Mar 9, 2015

chenw :

I will have to disagree. The whole point of going SLI is that you should be able to add another card down the line, or they'd make all their upper-tier cards hard-SLI-d like the 690(and we'd see GTX 665, 675, 685, etc with dual GPU variants of the corresponding GPUs).
With SLI, you do have to have a higher budget than when you don't plan to go SLI, and invest in your hardware accordingly. You'll have to get more powerful hardware that'll be the same throughout, such as power supply, CPU, motherboard. You'll also have to buy the cards smartly, such as getting the 6GB variant of the 780 or 4GB variant of the 770 if you plan to SLI.

Mortis Angelus · Mar 9, 2015

chenw :

cst1992 :

Hello and thanks for the reply.

If it is like what you just explained I don't see an issue wit using the Asus Z97 Pro Gamer for SLI setup. They even market it for quad-SLI (not that I would use that).

Neither do I understand the issue of having to go SLI straight away.Pro gamer mobo + 4790k + 750w 80+ gold PSU and 2400 mHz DRAM should handle SLI easy, shouldn't it?

I also know it is better to have higher VRAM, but I see plenty of examples out there with SLI/Xfire with normal GPUs even on really high-end rigs.

Btw: where did you find the info about the PCI-E x1 being chipset controlled? The mobo has two of them, and I wonder if both are chipset controlled.

And do all SATA eat up PCI-E lanes, or is it only the SATA Express and M.2 that uses PCI-E 2.0 lanes?

cst1992 · Mar 9, 2015

Only the M.2 and SATA-E.
You could save some money by going for a Seasonic M12II Bronze Evo and a i5 4690k. If you're into gaming, the i7 has no advantage apart from 500Mhz extra, but it should be easy enough to get on a 4690k anyway.
The Seasonic is a Bronze rated PSU, but has really high-end parts inside it. So it should handle 970/980 SLI just fine.

chenw · Mar 9, 2015

1. Regarding chipset, I cannot for the life of me remember where I got that information from, but all I know is that if all lanes all came from the CPU, then you wouldn't have been able to even install M.2 in an SLI setup, as that also eats up lanes. It could very well be that the 2.0 and 3.0 lanes both come from CPU, but they do not conflict with each other.

2. It supports Quad SLI meaning that you can SLI two Dual GPU cards together for pseudo-quad sli setup. The motherboard does not have enough lanes to even install 4 GPUs onto, let alone running them in SLI.

3. The issue I was addressing and cst1992 were disagree'ing upon wasn't about SLI itself, but rather there was any merit in opting to use that rig with 1 GPU initially, and then opting for SLI later as an upgrade, or just SLI straight away. It was a practicality argument, not a technical one. That setup is fully capable of running two GPUs in SLI, whether right away or down the line.

Mortis Angelus · Mar 9, 2015

cst1992 :

chenw :

Thanks again for the swift replies!

Off-topic: I'm gonna stick to the 4790K, because I do video and photo editing and especially the video editing demands quite a bit of resources. I also want my rig to be in the top-chart for a while, and while games today do not use more than four cores there's no saying what they will use in 2 - 3 years or so if you look at other software like video and photo editing software as games nowdays simulate more and more physics and other details.
Most likely I will never do SLI and just go single GPU, but I still want the option to do enthusiast (not high-end) SLI if I so desire.

As for the PSU I've chosen the Seasonic G-750 Semi-modular 80+ Gold. A bit more expensive, but should provide power more efficiently and last forever (like all Seasonics)

Back to the topic: Here's the link to the mobo-user manual: http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/Z97-PRO_GAMER/E9782_Z97-PRO_GAMER_Guide_web_only.pdf. It discusses PCI-ports on PDF-pages 28 - 30. Now as I understand those charts both the PCI-E x1 and PCI are separate from the PCI 3 x16 slots. And as chenw stated it would be stupid of Asus to market a mobo for SLI but put all PCI-ports on the same lanes.

All I basically want to know: Can I SLI/Xfire whilst also having a PCIx1 WLAN adapter? (And in the case of getting a disk with SATA-E or M.2 can they be added without destroying the max 16-lane setup?)

chenw · Mar 9, 2015

It would explicitly state that on the manual if the PCI 2.0 shared lanes with 3.0, but if you were paranoid, first buy a wlan card and install it, and check if your SLI setup survives, if it does, M.2 should be fine as well.

Mortis Angelus · Mar 10, 2015

chenw :

That is the issue right there: I have a GPU and I have WLAN-adapter. I don't have a SLI-config. And thus I want to know if this mobo is good enough for future upgrade to SLI or not and avoid wasting money in the process.

Stupid mobo-makers for not sharing this vital information, because it is not only Asus that won't share their PCI-E architecture.

cst1992 · Mar 10, 2015

1: M.2 Socket 3 shares bandwidth with PCIEX1_1 and PCIEX1_2 (in PCIE mode) & SATA6G_4 (in SATA mode), and supports M Key and type 2260/2280 storage devices.

This is all that is given on ASUS's website, so as long as you don't install a drive in the M.2 port and a card in any of the PCIe x1 slots, you should be fine.

chenw · Mar 10, 2015

Mortis Angelus :

Even better, here is how you check it:

Use GPU-Z, install only your GPU first, boot it up, and check if it runs on X16.

Then install your WLAN card, boot it up and check GPU-Z again, if the card is STILL on X16, then you are fine. if there is anything that competes with the CPU PCI-E lanes with the GPU, it should immediately jump to X8.

Mortis Angelus · Mar 10, 2015

chenw :

Thanks! That is a great idea, and most likely the best idea. However it doesn't solve the issue per se, because then I will have to buy the entire thing first before I know if it works or not, and that was the entire point of this question; to avoid buying the "wrong" mobo.

But thanks a lot anyway guys. I think I'll try to contact ASUS themselves about this. If God willing I'll get an answer in a month or two. Otherwise - never.

EDIT: Well f**k that: Either you have to have the product already (require serial number), or you have to be a business, or you have to live in US for call-service.... Wohoo... NOT

chenw · Mar 10, 2015

Well, if you are looking for alternative Motherboards, I can personally vouch that your WLAN card should be entirely compatible with SLI setup on Gigabytes Z97-UD5H, I have that exact motherboard, with PCE-AC68u and 2 970's in SLI.

However one thing I could caution is, depending on your WLAN card's heatsink thickness (if there is none, then ignore this comment), you might actually have to get a GPU without a backplate, as I was really close to learning that the hard way with PCE-AC68u, the heatsink prevented me from installing it directly above a GPU with a backplate, so I have to install the one without on the top (I use 1 Vanilla MSI 970 Gaming 4G, the other is Golden Edition). If the WLAN has no heatsink then you should be ok,

male-1950 · Jan 17, 2016

enemy1g :

Ok, the PCIe xXX is the amount of lanes the slot has. so x16 has 16 lanes, x8 has 8 lanes and so on.

The physical slot on x4, x8, and x16 are all the same.

If it says 2 PCIe x16 slots it means that there are 2 slots on the board that are capable of 16 lanes.

If it says 2 PCIe X16 (and then in brackets says x16, x4 or x8, x8) it means when you have 1 slot being used it uses all 16 lanes for that 1 slot. But if you plug multiple things into the PCIe slots (most common 2 GPU's) it will divide the lanes between the cards. So that (x16, x4) you will have the first GPU running with 16 lanes, and the second GPU running with 4 lanes.

The manual for any board is going to say what the slot is at natively (x16, x8, x4) and how it reacts if in SLI (multiple GPU's)

For GPU's most of the time there is not much difference between 16x and 4x because GPU's can not fully saturate the bandwidth of the slot, therefore it is hardware limited, not bus limited.

But still, plug it in x16.

Shanm16 · May 31, 2016

gin23 :

Msi big bang marshal b3 with 67 chipset will support dual x 16 pci slot sli or crossfire as you like. Dual Gpu setup one gpu in first slot and second gpu in 5th slot both card will work with x 16 speed

Whats best for sli PCI-E x16 both or x16/x8

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