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SLI/PCIe question

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captainthrall

Member
Joined
Jun 24, 2016
I'm researching a PC build and I'm a little confused about how the functionality of PCI slots changes when using multiple cards.

Most newer boards have several PCI slots. The primary is usually PCI x 16.

However, if using 2x PCI slots (with dual video cards for example) then the speed of the primary slot drops to x8 right? Can anyone explain how and why this happens? How much does this affect the speed of the primary-slotted video card? I assume it would still be beneficial to have two cards rather than one right?

Does using 2x m.2 slots in raid also slow the speed of PCI slots?
 

ATMINSIDE

Sim Racing Aficionado Co-Owner
Joined
Jun 28, 2012
This depends from board to board and platform to platform... what specific CPU and motherboard are you asking about?
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
This depends from board to board and platform to platform... what specific CPU and motherboard are you asking about?

I haven't completely decided yet. I'm still weighing a few different options, but right now I'm leaning towards a skylake i7-6700k CPU with either of these two motherboards:

-MSI Gaming Z170A XPOWER GAMING TITANIUM EDITION LGA 1151 Intel Z170 HDMI SATA 6Gb/s USB 3.1 ATX Intel Motherboard
http://www.newegg.com/Product/Produ...870&cm_re=msi_titanium-_-13-130-870-_-Product

-EVGA Z170 Classified K 142-SS-E178-KR LGA 1151 Intel Z170 HDMI SATA 6Gb/s USB 3.1 USB 3.0 ATX Intel Motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16813188181
 

EarthDog

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As ATM said.. it depends (on information not listed).

AS far as M.2 slots, at least with Z170/X99 these are fed from the PCH/chipset and have no bearing on the PCIe slots/bandwidth.


EDIT: Just saw your post... unless you are going sub-ambient cooling and overclocking that heavily, you can easily run a $150 board and overclock the snot out of that CPU. Dont pay a premium for things you won't use. ;)

Any reason you are looking for SLI out of the gate? Unless you are going 4K, there typically isn't a good reason for the additional hassle.
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
As ATM said.. it depends (on information not listed).

AS far as M.2 slots, at least with Z170/X99 these are fed from the PCH/chipset and have no bearing on the PCIe slots/bandwidth.


EDIT: Just saw your post... unless you are going sub-ambient cooling and overclocking that heavily, you can easily run a $150 board and overclock the snot out of that CPU. Dont pay a premium for things you won't use. ;)

Any reason you are looking for SLI out of the gate? Unless you are going 4K, there typically isn't a good reason for the additional hassle.

My goal is to build a system that has good performance and looks nice. I chose the two boards I listed because they're capable of running 2x M.2 slots in RAID 0 - and they look awesome (this is subjective of course).

I'm pushing 3x monitors, but still probably don't NEED to go SLI out of the gate... though I'd like to be able to rock 60+fps @2k+ in newer games. I'm not entirely sure if I'll go SLI, but I'd like to research more about it before I make a decision.

I've pre-ordered a case (evga DG-87) that wont be shipping until August, so I have a full month to figure out exactly what to jam inside of it. That's why I'm researching at the moment.
 

EarthDog

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1. Do you have a need for 2x M.2 in R0? If not, its OK to want and have it of course, but really, one of these NVMe based drives are fast enough for 99% of the population. Make sure you are in the 1% as the cost really isn't worth the performance unless you deal with large file transfers quite a bit. Not to mention you are doubling the chance at failure (2 pieces of hardware... in R0 one goes down it is all down).
2. 2K+? What res is 2K? Are you 1920x1080 x3? Or you 2560x1440 x3? A little confused there.
 
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captainthrall

captainthrall

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Joined
Jun 24, 2016
1. Do you have a need for 2x M.2 in R0? If not, its OK to want and have it of course, but really, one of these NVMe based drives are fast enough for 99% of the population. Make sure you are in the 1% as the cost really isn't worth the performance unless you deal with large file transfers quite a bit. Not to mention you are doubling the chance at failure (2 pieces of hardware... in R0 one goes down it is all down).
2. 2K+? What res is 2K? Are you 1920x1080 x3? Or you 2560x1440 x3? A little confused there.

Ahah, I definitely don't need to raid 2x M.2 SSDs, but I'm not looking to build an economical rig. If I was doing that, I'd pick up a mid-level AMD prebuild from the super-sale section. I've read that Samsung M.2s have a much lower failure rate than classic spinning HDDs and even a significantly lower failure rate than standard SATA SSDs... so I'm not really worried about the risk of RAID failure.

2k resolution is between 1080 and 4k. 720= 1280x720 pixels. 1080 = 1920x1080 pixels. 2k = 2048x1080(sometimes it's 2048x1556, I've even seen 2560x1440 called 2k). 4k = anywhere from 3840x2160 to 4096x2160 (ya, it's weird)

Basically I'm shooting for 2048x1080 or better resolution on all 3 monitors (currently one is 32 inches and two are 24 inches. I'm probably going to buy new monitors to go 32''+27''+27'')

Right now I'm hoping to learn more about PCI slots and how their bandwidth is restricted while multiple slots are occupied (and why).
 
Last edited:

EarthDog

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There is a big difference between a store bought PC and what you are trying to build (saying there is a happy medium between cost, performamce, and reliability).. i digress. :)


What more do you need to know about the slots? It depends where those slots are fed from on those boards. Typically one is the ultra fast 32gbps while the other is 10gbps so you would neuter the other drive regardless of raid. Make sure the boards have 2 full speed slots.

I asked about "2k" res as i wasnt sure if you had 2048x xxxx monitors... thats a weird arse res. If you are going 5760x1080 (3 1920x1080 monitors) a si gle 1080 would be fine, or maybe wait for the 1080ti.
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
Most newer boards have several PCI slots. The primary is usually PCI x 16.

However, if using 2x PCI slots (with dual video cards for example) then the speed of the primary slot drops to x8 right? Can anyone explain how and why this happens? How much does this affect the speed of the primary-slotted video card? I assume it would still be beneficial to have two cards rather than one right?
 

ATMINSIDE

Sim Racing Aficionado Co-Owner
Joined
Jun 28, 2012
Most newer boards have several PCI slots. The primary is usually PCI x 16.

However, if using 2x PCI slots (with dual video cards for example) then the speed of the primary slot drops to x8 right? Can anyone explain how and why this happens? How much does this affect the speed of the primary-slotted video card? I assume it would still be beneficial to have two cards rather than one right?

Number of lanes available determine if the slot drops.
Boards do not drop the speed if they have a PLX chip.

2-3% hit to the GPU, at most.

Assuming you'll be pushing the GPU hard enough to justify a second GPU, yes.
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
Number of lanes available determine if the slot drops.
Boards do not drop the speed if they have a PLX chip.

2-3% hit to the GPU, at most.

Assuming you'll be pushing the GPU hard enough to justify a second GPU, yes.

Alright, so the difference between x16 and x8 doesn't really matter much? Good to know
 

EarthDog

Gulper Nozzle Co-Owner
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Dec 15, 2008
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Most newer boards have several PCI slots. The primary is usually PCI x 16.

However, if using 2x PCI slots (with dual video cards for example) then the speed of the primary slot drops to x8 right? Can anyone explain how and why this happens? How much does this affect the speed of the primary-slotted video card? I assume it would still be beneficial to have two cards rather than one right?
heres link..about 1%: https://www.techpowerup.com/reviews/AMD/R9_Fury_X_PCI-Express_Scaling/18.html

Yes, its beneficial. Scaling is from say 50-80% when working properly. It dkesnt always work properly. ;)
 
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captainthrall

captainthrall

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Joined
Jun 24, 2016
heres link..about 1%: https://www.techpowerup.com/reviews/AMD/R9_Fury_X_PCI-Express_Scaling/18.html

Yes, its beneficial. Scaling is from say 50-80% when working properly. It dkesnt always work properly. ;)

Oh man, thank you very much for that link. The data presented definitely tells the full story. Not only was I surprised by the meager 1% loss when dropping from x16 to x8, I was blown away to see that dropping from PCIe x16 3.0 to PCIe x16 2.0 only dropped performance by 1% as well. For some reason, I expected the difference to be huge.

x16 --> x8 = 1% loss in performance
pci 3.0 --> pci 2.0 = 1% loss in performance.

I also found this very informative video explaining PCI slots: https://youtu.be/rctaLgK5stA

Basically he explains that pci "lanes" are basically just bandwidth and different CPUs have different numbers of lanes. Most current generations of chips have 16 lanes, 28 lanes, or 40 lanes. Obviously if you're building a pc with 4-way-SLI or any type of setup that requires a lot of PCI connectivity, it's very important to have a processor that has enough lanes (bandwidth) to feed everything, otherwise you're just going to get bottlenecked.

Personally, I wasn't sure if I wanted to go with an i7-6700k, or an i7-6800k. They're similar in benchmarks. One has 2 more cores, but the other has a 25% faster clock speed - so they kind of balance out. The real kicker is: the 6700 only has 16 PCI lanes while the 6800 has 28.
http://ark.intel.com/products/88195/Intel-Core-i7-6700K-Processor-8M-Cache-up-to-4_20-GHz

Obviously having 16 pci lanes is fine if you're only running 1x graphics card. You get the maximum x16 speed. Even if you're running dual SLI, you're only losing 1% per card (according to the benchmarks Earthdog posted). No problem, but if you're running triple SLI, you're going to be downgrading your PCI speeds to 1x8+2x4 take a hit of roughly 5% per card on the slower slots. I'm not sure if that would result in a cumulative effect of 5% + 5% + 1% = 11% or just a total of 5% loss in performance. And then obviously if you want to go quad-SLI, you'd have to get a different chip, because according to intel, the 6700k doesn't support it.

These are all very interesting things for anyone pondering an SLI build to consider.

I have another question. Do M.2 slots draw from PCI lanes as well? I know the old, lower speed slots were connected to SATA ports somehow, but I'm under the impression that the new M.2 ultra x4 slots draw from PCI bandwidth. Can anyone confirm or explain this?
 

EarthDog

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Yes, but very few (sane) people go triple cards. The more cards you add, the more scaling gets to be bad. Where you are seeing 50-90% or so with dual cards, adding a third may only yield another 30-50% more, four cards is even less. If you are worried about losses, going more than two cards is going to be your biggest concern. You also don't need it. You also cannot run SLI on 4x, it has to be 8x. CFx (AMD) you can however. Definitely interesting, but definitely something known for years as well :)

M.2 on Z170 is from the chipset/PCH, not the CPU PCIe lanes.
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
I just read that Nvidia requires a PCIe x8 or faster lane in order to run SLI configurations, so doing 3-way SLI on a 16 lane processor is not possible

EDIT: ahah, yeah you beat me to it. Thanks for the info Earthdog!
 

ATMINSIDE

Sim Racing Aficionado Co-Owner
Joined
Jun 28, 2012
I just read that Nvidia requires a PCIe x8 or faster lane in order to run SLI configurations, so doing 3-way SLI on a 16 lane processor is not possible

EDIT: ahah, yeah you beat me to it. Thanks for the info Earthdog!

If you buy a board with a PLX chip it is possible.
Again, you don't need three cards though.
 

EarthDog

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Its possible if the motherboard has a PLX chip (created more PCI lanes)... which not many do.

EDIT: What he said again! :rofl:
 
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captainthrall

captainthrall

Member
Joined
Jun 24, 2016
If you buy a board with a PLX chip it is possible.
Again, you don't need three cards though.

yeah, there's a 0% chance that I will go 3-way-SLI. I'm just curious to learn how it all works.
Here's another scaling benchmark showing the difference between 3.0/2.0, x16/x8, etc (this time with Nvidia and in FPS rather than percentages)
https://www.techpowerup.com/reviews/NVIDIA/GTX_980_PCI-Express_Scaling/20.html

I also read this:

The z170 board have 20 lanes connected to the chipset and 16 lanes with the cpu, so the m.2 should be connecting with the chipset, not taking away any bandwith from the card, if you have all the chipset lanes used up then it could put your gpu in 8x, but that will not affect you graphics card preformance anyway.


Which things connect to the chipset lanes, and which connect to the cpu lanes? is it...
Chipset = SATA/USB/M.2
CPU = PCI only
?