More PCIe slot confusion

[needgpu47]

I read the posts about PCIe slots and I read my motherboards manual. The manual is confusing about a point and it doesn't explain it clearly.

I purchased an Aorus X299 Gaming 7. You can find the manual here: http://download.gigabyte.us/FileList/Manual/mb_manual_ga-x299-aorus-gaming-7_e.pdf

On page 21 it explains how to install GPU's and the diagram shows 5 PCIe slots that are physically each 16 lanes. Two things are confusing:

1. The slots are labeled in such a way that, even though they are all physically 16 lanes, it's implying that not all slots support 16 lanes. Is that true? If I want to put one and only one GPU into slot 2 will it only be 4 lanes?

2. Slot 4 isn't labeled. Does that mean it doesn't work?

 

[atljsf]

yes, that is correct

depending on the cpu you install, some of the slots will not run a x16 thanks to the pci express lanes available to communicate cpu with the pci express slots

this becomes very reall when you buy a 4 core x299 cpu which only offers 24 lanes, gpu using 16, sata ports using other ports

there you see how fast those lanes start to get occupied, if you put a m.2 nvme, more lanes will get occupied and some sata ports will not be available to use

what parts do you have there to use with the motherboard?

more than 1 gpu?

what cpu did you bought?

 

[needgpu47]

"yes, that is correct" -> Which is correct? That if I I only put one GPU into slot 2 that it will only be 4 lanes? That slot 4 doesn't work at all?

I get that CPUs have a differing numbers of lanes. I'd like to understand the first principles here, so I phrased those two questions to be CPU agnostic. Given the available lanes from the CPU, does the motherboard assign lanes dynamically where ever I put the cards or will slot 2 only every be 4 lanes even though it can accept a 16-lane card. If you look at the manual, page 21, my question might seem more clear.

 

[atljsf]

in the x299 the cpu matters alot, the better the cpu, more lanes available, the smaller, talking about the joke 7740 cpu with 4 cores, the less lane available

the first assumption you made is correct but depends on the hardware used

 

[needgpu47]

OK, so if I put only 1 GPU in and I put it in slot two, it will only have 4 lanes available regardless of the fact that the CPU may be overflowing with available lanes and slot 2 can accept a 16-lane card. I can accept that if you're telling me it's true.

But what about slot 4? Does it really have no lanes? Why even put it on the motherboard.

 

[atljsf]

i never said that a specific slow will have a spacific ammount of lanes available depending on the cpu, what i said is that the first slot used for a gpu usually gets 16 lanes for it, if you put two gpus that number usually drops to 8 lanes per slot

things have been like this for many years, x299 is not inmune to that

i mentioned the type of cpu becuase only the 10 cores or more cpus will have more pci lanes to comunicate with devices


i asked you clearly to tell me what cpu you will use to know what limitations will find on the motherboard but you kept complaining about pci limitation that exist

always remember that you can put a certain ammount of devices on a certain amount of slots and the sysem will do the best to handle them as long as the re is comunication lanes between the cpu and the devices via the motherboard

each manufacturer decides what to implement and how to handle cpu limitations, there is not a standard rule to follow

the cpu question i made to you, some manufacturers are doing motherboard that if you put the 7740, some of the motherboard ramm slots will be disabled and will not be used or seen by the system

same can apply for m.2 slots, sta ports, pci slots

the only limitation that has been shared for many years is when you put two gpus and you have only 20 pci lanes or less, the first gpu when alone gets 16 lanes if available, if there is two both get 8

if you are on a budget platform your gpu might get only 4

cpu and motherboard will choose how to communicate with each other the best way they can