Query on pciE lane usage

[forteh]

Hi all, having a real brainfart with my pciE lanes and slots and hoping you can clear up my stupidity!

I have a 5800x3D on an msi B450M mortar max with a 3080ti and an intel 750 pciE nvme drive as the primary boot drive (with 3xSATA drives for storage)

Specs are below: -

My query is, am I forcing the GPU slot to run at pciE 3x8 because I have the intel SSD in the pciE 2x16 slot?

My intention is to get an M2 drive at some point in the near future but I want to know if I should be looking to ditch the intel drive if it's likely to be slowing the GPU down. From what I've read the ryzen has 20 pciE lanes, the GPU uses 16 and the NVME uses 4 so if I were to add an M2 drive in addition to the intel would it then cripple the system elsewhere?

 

[mickyflinn]

The intel pcie drive in the lower gpu is making your Gpu run at x8 instead of x16 because the drive needs to share lanes.

Get rid of the intel drive and get a m2 nvme drive and use the correct m2 slot , your motherboard is limited to pcie3 as its a b450 . B550 and x570 motherboards support pcie4

 

[forteh]

Thanks, that's pretty much as I suspected (not that I'm getting bad performance from the 3080ti), the intel drive is a hangover from my old 2500k system and is slower than most pciE3 nvme drives so its days were likely numbered anyways.
I presume the M2 slot next to the GPU is the preferred one to use?

I primarily use the machine for DCS vr so the most throughput I can get the better!

 

[mickyflinn]

Thanks, that's pretty much as I suspected (not that I'm getting bad performance from the 3080ti), the intel drive is a hangover from my old 2500k system and is slower than most pciE3 nvme drives so its days were likely numbered anyways.
I presume the M2 slot next to the GPU is the preferred one to use?

I primarily use the machine for DCS vr so the most throughput I can get the better!

Yes above the gpu but just check .

You could get s pcie4 m2 as they are backwards compatable but you will be limited to pcie3 speed.

What size you after ?

 

[forteh]

The intel is 400gb so ideally no smaller than that, I would probably go for 1tb though - hoping for something coming up in the sales soon

edit: I'm guessing that one M2 slot uses CPU lanes and the other M2 slot uses chipset lanes? Would I be better to utilise the chipset ones over the CPU ones, the manual states that if I put an M2 in slot2 then it disables one of the pciEx1 slots. Would using the chipset lanes then take away bandwidth from the USB etc?

 

[alec]

Hi all, having a real brainfart with my pciE lanes and slots and hoping you can clear up my stupidity!

I have a 5800x3D on an msi B450M mortar max with a 3080ti and an intel 750 pciE nvme drive as the primary boot drive (with 3xSATA drives for storage)

from https://www.msi.com/Motherboard/B450M-MORTAR-MAX/Specification

• 1 x PCIe 2.0 x16 slot (PCI_E4, supports x4 mode)1

(1) PCI_E4 slot will be unavailable when an M.2 SSD is installed in the M2_2 slot

There is no interaction with main x16 slot.

Does GPUZ show your graphics card run in x8 mode?

 

[forteh]

from https://www.msi.com/Motherboard/B450M-MORTAR-MAX/Specification

• 1 x PCIe 2.0 x16 slot (PCI_E4, supports x4 mode)1

(1) PCI_E4 slot will be unavailable when an M.2 SSD is installed in the M2_2 slot

There is no interaction with main x16 slot.

Does GPUZ show your graphics card run in x8 mode?

Perhaps this is partly the cause of my confusion, I'm not sure exactly which slots are using which lanes and which are sharing and which are not!
Stupidly I haven't checked the simple stuff like gpuz Will do so after work

 

[mickyflinn]

Perhaps this is partly the cause of my confusion, I'm not sure exactly which slots are using which lanes and which are sharing and which are not!
Stupidly I haven't checked the simple stuff like gpuz Will do so after work

Looked at this again because the motherboard supports crossfire I thought using the intel m2 in pcie_E4 would share the pcie3 lanes from the gpu . Crossfire can be supported threw the chipset so therefore It looks like that slot is conected by the chipset .

Sorry for the confusion.

 

[Tetras]

Perhaps this is partly the cause of my confusion, I'm not sure exactly which slots are using which lanes and which are sharing and which are not!
Stupidly I haven't checked the simple stuff like gpuz Will do so after work

The second full length PCI-E slot has 4 lanes (from chipset) which are shared with the second M.2 slot. If the second M.2 slot is used, then the PCI-E slot doesn't work.

PCI-E slot 2 and 3 (the little ones) also share lanes.

 

[forteh]

Looked at this again because the motherboard supports crossfire I thought using the intel m2 in pcie_E4 would share the pcie3 lanes from the gpu . Crossfire can be supported threw the chipset so therefore It looks like that slot is conected by the chipset .

Sorry for the confusion.

No issues at all, as I said it's probably my next upgrade

The second full length PCI-E slot has 4 lanes (from chipset) which are shared with the second M.2 slot. If the second M.2 slot is used, then the PCI-E slot doesn't work.

PCI-E slot 2 and 3 (the little ones) also share lanes.

So I should be able to use the top M2 slot next to the CPU without any detriment to GPU or chipset performance? In turn technically I could run both the top M2 (using CPu lanes) and the intel SSD (using chipset lanes) concurrently, alternatively run a pair of M2 drives if I ditch the intel SSD from slot 4.

I don't have anything that will fit into slots 2 & 3

 

[mickyflinn]

No issues at all, as I said it's probably my next upgrade



So I should be able to use the top M2 slot next to the CPU without any detriment to GPU or chipset performance? In turn technically I could run both the top M2 (using CPu lanes) and the intel SSD (using chipset lanes) concurrently, alternatively run a pair of M2 drives if I ditch the intel SSD from slot 4.

I don't have anything that will fit into slots 2 & 3

Correct

But the second m2 runs at pcie2.0 x 4 half the speed of the other which equates to 1500 mbps 3 times as fast as a standard ssd.


AMD® B450 Chipset4 x SATA 6Gb/s ports11 x M.2 slots (M2_2, Key M)1Supports PCIe 2.0 x4 2242/ 2260 /2280 storage devices

 

[forteh]

Well the answer is, it's running at pciE 3.0x16 according to gpuZ , really don't know why I didn't check that first!
Either way you guys have been very helpful will helping me understand the pciE lanes and how they interact with one another, now to wait for a decent sale on an M2

 

[Ryder_1990]

So I'm trying to figure out if I can keep 16 lanes for my GPU. As Zen 4 has 24 usable lanes then 16 are for GPU, 4 for M.2 NVMe no1, 4 for M.2 NVMe no2. As per manual for ROG Strix X670-E A:

• AMD Ryzen™ 7000 Series Desktop Processors:
- M.2_1 supports PCIE 5.0 x4 mode M Key design and type 2242 / 2260 / 2280
storage devices.
- M.2_2 supports PCIE 5.0 x4 mode M Key design and type 2242 / 2260 / 2280
storage devices.
• AMD X670 Chipset:
- M.2_3 supports PCIE 4.0 x4 mode M Key design and type 2242 / 2260 / 2280 /
22110 storage devices.
- M.2_4 supports PCIE 4.0 x4 mode M Key design and type 2242 / 2260 / 2280
storage devices.

To me if I plug in two NVMe SSDs in top two slots then I shouldn't lose anything from GPU as I will just fill up 24 lanes. I see no reason then why GPU would have to slow down to x8 mode or is that too simple?

 

[Tetras]

To me if I plug in two NVMe SSDs in top two slots then I shouldn't lose anything from GPU as I will just fill up 24 lanes. I see no reason then why GPU would have to slow down to x8 mode or is that too simple?

Correct. Intel boards are having to do this with PCI-E 5.0 M.2 slots, because there are no spare lanes (only 16), X670E doesn't have this problem.