PCIe lane splitup for m.2/cpu/gpu

[WinterLFG]

I'm upgrading to a 3900XT CPU later this year (Most likely will switch to the 4000 CPUs when they come out).  In terms of PCIe lanes, the 3900XT has 20 lanes, obviously my GPU takes 16x lanes.

 

If I buy an M.2 drive, will the lanes be split weirdly?  16x/4x, 10x/10x, 12x/8x or anything like that?

Also the Mobo I'm buying is a X570 and has 2 M.2 slots.  What would happen to the lanes then?

 

Thanks

[-rascal-]

3 minutes ago, WinterLFG said:

I'm upgrading to a 3900XT CPU later this year (Most likely will switch to the 4000 CPUs when they come out).  In terms of PCIe lanes, the 3900XT has 20 lanes, obviously my GPU takes 16x lanes.

 

If I buy an M.2 drive, will the lanes be split weirdly?  16x/4x, 10x/10x, 12x/8x or anything like that?

Also the Mobo I'm buying has 2 M.2 slots.  What would happen to the lanes then?

 

Thanks

 

Depends on which Mobo you get.

On X570 or B550, the top M.2 slot is the extra 4 PCI-E lanes coming from the CPU.

So..split 16 + 4.

 

Some boards will have the 2nd (and 3rd) M.2 slots being PCI-E 3.0 from the chipset, or PCI-E 4.0 from the chipset.

[Skiiwee29]

every CPU will have 20 lanes for PCI devices. So in this case, 1 16x or 2x8x and 1 x4 for an m.2 SSD. The rest are given to the board manufacture to split and do with what they please for the most part with the Chipset and I/O 

[fordy_rounds]

This depends on your motherboard, but, as I understand it, it's usually pre-split (by default) as 16 to the GPU, 4 to the main M.2. Your bios may allow you to split the 16 down to 8 and hook the extra 8 to the next slot down; this is particularly useful for SLI/Crossfire setups, though that's certainly not the only use case.

The second M.2 slot (as well as the PCIe slots other than the top one, or top two if they can be split) is often hooked to the chipset rather than direct to the CPU. (The chipset can provide a whole bunch of extra lanes, and basically acts as a switch/multiplexer to connect them to the CPU.)

[WinterLFG]

7 minutes ago, fordy_rounds said:

This depends on your motherboard, but, as I understand it, it's usually pre-split (by default) as 16 to the GPU, 4 to the main M.2. Your bios may allow you to split the 16 down to 8 and hook the extra 8 to the next slot down; this is particularly useful for SLI/Crossfire setups, though that's certainly not the only use case.

The second M.2 slot (as well as the PCIe slots other than the top one, or top two if they can be split) is often hooked to the chipset rather than direct to the CPU. (The chipset can provide a whole bunch of extra lanes, and basically acts as a switch/multiplexer to connect them to the CPU.)

Okay, the Mobo I will be using is a X570.  Just to be clear, the second M.2 slot will not effect the CPU lanes? wouldn't it be a little slower as well because of this?

[Skiiwee29]

57 minutes ago, WinterLFG said:

Okay, the Mobo I will be using is a X570.  Just to be clear, the second M.2 slot will not effect the CPU lanes? wouldn't it be a little slower as well because of this?

The 2nd m.2 will be ran through your chipset at PCI Gen 3x4 instead of PCIe Gen 4 like the main one will be. 

[WinterLFG]

12 minutes ago, Skiiwee29 said:

The 2nd m.2 will be ran through your chipset at PCI Gen 3x4 instead of PCIe Gen 4 like the main one will be. 

Okay, thank you!

[fordy_rounds]

15 hours ago, Skiiwee29 said:

The 2nd m.2 will be ran through your chipset at PCI Gen 3x4 instead of PCIe Gen 4 like the main one will be. 

On an x570 it would be PCIe Gen 4x4, not Gen 3x4 (if the SSD supports Gen 4).

However, depending on what else is in use, you may still see slightly slower speeds due to the chipset multiplexing the Gen4x4 connection to the CPU. It will not affect the CPU lanes at all. (In practical terms, most devices don't usually saturate their PCIe link, especially over PCIe Gen 4; if you're benchmarking it could saturate and you might see some slowdown, but in normal use it won't be noticeable.)

[Skiiwee29]

4 hours ago, fordy_rounds said:

On an x570 it would be PCIe Gen 4x4, not Gen 3x4 (if the SSD supports Gen 4).

However, depending on what else is in use, you may still see slightly slower speeds due to the chipset multiplexing the Gen4x4 connection to the CPU. It will not affect the CPU lanes at all. (In practical terms, most devices don't usually saturate their PCIe link, especially over PCIe Gen 4; if you're benchmarking it could saturate and you might see some slowdown, but in normal use it won't be noticeable.)

Chipset m.2 speeds on x570 are only gen 3, not gen 4. its Gen 4 direct to the CPU if I remember correctly. 

[BabaGanuche]

2 hours ago, Skiiwee29 said:

Chipset m.2 speeds on x570 are only gen 3, not gen 4. its Gen 4 direct to the CPU if I remember correctly. 

No the x570 chipset lanes are 4.0 the b550 is 3.0 chipset lanes.

 

Chipset differences

[Skiiwee29]

19 minutes ago, BabaGanuche said:

No the x570 chipset lanes are 4.0 the b550 is 3.0 chipset lanes.

 

Chipset differences

ahhh, ok.. probably got mixed up since the B550 reviews and stuff and my brain thinking of that.