x299 w/7640x complete confusion (not a rant)

[Allegrif]

First off this is my first post on LTT as a relatively recent PC convert, so uh... hi.

 

I need something clearing up in my head, not because I'm thinking of buying (I'm fancying Threadripper) but just because I can't get my head around it and the numerous other sources I've read seem to give completely conflicting information. 

 

x299 motherboards are ideally built with 44 PCIe lanes in mind, so I know putting a 16 lane CPU in there is going to disable a lot of features. My confusion comes from the additional chipset lanes. If a board has 3 M.2 slots, and is configured to give all 16 lanes of a 7640x/7740x to the top expansion slot (GPU for example), does this render the M.2 slots completely useless, or are they still connected through the chipset? If they are connected through the chipset, would putting 3 M.2 SSDs in RAID 0 be bottlenecked by said chipset? 

 

Further to this, if you went and put a 44 lane CPU in later, would this allow the M.2 slots to connect straight to the CPU rather than the chipset, or are these slots always going to use the chipset regardless of CPU lanes available? 

 

So much confusion on this platform as a newbie it's unreal 

[Lurick]

As far as I understand (So I might be wrong) the CPU lanes are directly wired into the CPU socket itself giving lower latency and faster communication with the CPU itself. The chipset lanes go through the southbridge and up to the CPU adding some additional latency but still having the full bandwidth. So if you have a chipset that provides 16 or 20 lanes and a CPU that provides 16 lanes then your GPU would get 16 lanes from the CPU and the M.2 slots would get their lanes from the chipset but wouldn't be bottlenecked exactly but there might be a few nanoseconds of added latency.

 

It also depends on how the board is wired. Sometimes there might be an M.2 slot wired to a physical PCIe slot and would thus make use of those lanes instead of the chipset lanes.

 

To add onto that even the 1080Ti doesn't need the full x16 lanes, you could put two GPUs in there and each would operate at x8 (PCI 3.0 of course) and still be fine. You're talking maybe 1 FPS difference.

 

 

Edit:

Just to put it out there, for a newbie to the PC side of things, you seem like you've got a solid grasp on stuff so far  

[ItsTheDuckAgain]

Im not that hard into it but maybe this helps you out a bit:

[Allegrif]

Thanks guys, so the CPUs having 16 lanes isn't actually as horrible as it's made out to be, as most people don't realise the chipset provides a bunch of other lanes anyway for the M.2s etc? If I'm understanding this right, the main concern with only having 16 CPU lanes is less expandability in terms of traditional physical PCIe slots for 3x GPU setups or 2x GPU plus other expansion cards etc?

 

And as far as the M.2 slots go, it depends on the board as to whether a 44 lane CPU would allow the slots to use the CPU lanes or still restrict to the chipset lanes? 

 

And everything I do know I've basically just learned from watching Linus and a couple of others like Gamers Nexus on YouTube 

[The Benjamins]

To really know how a board will work you will have to download the manual and read it to find how the RAM, PCIe, sata, M.2, USB is effected by CPU choice and stuff.

 

I personally do not recommnd  a 7640X or 7740X at all.

 

He goes over 3 Motherboards and what is effected if using the 3 tiers of CPU's.

 

I would wait for X399 TR, if it also has a "low core" part like a 4c or 8c part that could fit your needs. (besides High FPS gaming) BUT it will have 64 PCIe lanes so you can throw GPUs and NVME drives at it with out worrying about it.

[Lurick]

4 minutes ago, Allegrif said:

Thanks guys, so the CPUs having 16 lanes isn't actually as horrible as it's made out to be, as most people don't realise the chipset provides a bunch of other lanes anyway for the M.2s etc? If I'm understanding this right, the main concern with only having 16 CPU lanes is less expandability in terms of traditional physical PCIe slots for 3x GPU setups or 2x GPU plus other expansion cards etc?

 

And as far as the M.2 slots go, it depends on the board as to whether a 44 lane CPU would allow the slots to use the CPU lanes or still restrict to the chipset lanes? 

 

And everything I do know I've basically just learned from watching Linus and a couple of others like Gamers Nexus on YouTube 

 

Pretty much. There are a lot of boards though that wire at least 1 M.2 slot into the PCIe slots so that's why you'll see something that says "PCIe slot X will be disabled if M.2 drive is populated" so in that case those would take CPU lanes but even then you can still run a single GPU at x8 speed without a problem.

[typographie]

1 hour ago, Allegrif said:

And as far as the M.2 slots go, it depends on the board as to whether a 44 lane CPU would allow the slots to use the CPU lanes or still restrict to the chipset lanes? 

I'm not certain, but I suspect it's not so simple as an NVMe suddenly doubling or quadrupling its performance just because it has access to 8 or 16 lanes via the CPU. The current generation of NVMe SSDs is designed around the notion that they're going to go into an M.2 slot getting 4 PCIe 3.0 lanes from the chipset, and they might not be able to scale much beyond that.

 

So while in theory I suppose a motherboard manufacturer could choose to that, I don't know if there's even a point. X299 still has 24 PCIe lanes limited to 4x, and I don't see any reason why they wouldn't use those.