Question Question about CPU PCIe lanes

[Havik IV]

I wanted to know if CPU lanes are distributed to components based on priority or are they specified by the motherboard manufacturer? For example, Intel i9 9820x has 44 lanes , and these lanes will first be given to the slotted GPU(s). A single GPU will use a maximum of 16 out 44 lanes, leaving 28 lanes. My question is will all of these 28 lanes used by distributing them to the other components in the PC based on priority level, NVMe instead of SATA, or will only some of the lanes be used by specific slots on the motherboard while the rest of the components going to the chipset?

 

[kanewolf]

PCIe lanes are pre-allocated at design time. So if you don't have two graphics cards, those 16 lanes in the second slot in your example are unused.

 

[AllanGH]

Most contemporary consumer graphics subsystems are allocated 16 PCIe lanes--total. If you populate the second "PCIe x16" slot, the 16 lanes are shared between the two GPUs--8 lanes per GPU. Similarly, if you toss 4 GPUs onto a MB, then each GPU will be utilizing 4 lanes.

Graphics subsystem PCIe lanes will go unused when you install a single GPU that does not utilize all 16 lanes.

There are, of course, exceptions to this, but they are not in the current majority case.

 

[Havik IV]

I think my question was misunderstood. In my example, I specified that the processor, 9820x, has 44 PCIe lanes in total. From what I read online about it, it does allow GPUs in x16, x16x16, and even x16x16x16 configurations. My question isn’t related to the GPU configurations, but in regards to how the remaining 28 out 44, in case of one GPU, lanes are used. Are they allotted to other components in the system using some type of proirity system/algorithm or are they only used when specific slots on the motherboard are used? For example will the remaining 28 lanes be given to LAN, M.2, SATA, RAM etc until all are used, allowing these components to connect directly to the CPU instead of all being routed through the chipset’s single x4 connection? Or will they only be allotted to specific component slots on the motherboard?

 

[AllanGH]

Good question, and I apologize for my misunderstanding.

I will, of course, have to dig-in and do some research on that.

But, there has to be a variance between reality and the PCIe lane count, as a x16-x16-x16 configuration for a graphics subsystem would exceed the number of available PCIe lanes....so, something is up, there.

 

[Havik IV]

Sorry I meant x16x16x8.

 

[kanewolf]

I think my question was misunderstood. In my example, I specified that the processor, 9820x, has 44 PCIe lanes in total. From what I read online about it, it does allow GPUs in x16, x16x16, and even x16x16x16 configurations. My question isn’t related to the GPU configurations, but in regards to how the remaining 28 out 44, in case of one GPU, lanes are used. Are they allotted to other components in the system using some type of proirity system/algorithm or are they only used when specific slots on the motherboard are used? For example will the remaining 28 lanes be given to LAN, M.2, SATA, RAM etc until all are used, allowing these components to connect directly to the CPU instead of all being routed through the chipset’s single x4 connection? Or will they only be allotted to specific component slots on the motherboard?

No. The SATA, network and other peripherals use PCIe lanes from the motherboard chipset. The M.2 slots might be CPU or chipset PCIe it would depend on the motherboard.

 

[AllanGH]

So, in short, you're looking at arbitrated PCIe lane access.....

 

[Havik IV]

Ok so it will depend on the motherboard meaning the remains of the lanes will be wasted with one GPU and two M.2 x4 NVMe drives installed.

So, in short, you're looking at arbitrated PCIe lane access.....

I’m not sure what that means.

 

[kanewolf]

Ok so it will depend on the motherboard meaning the remains of the lanes will be wasted with one GPU and two M.2 x4 NVMe drives installed.

I’m not sure what that means.

I would say "unused" rather than wasted. A specific CPU isn't chosen because of PCIe lanes alone.

 

[Havik IV]

I would say "unused" rather than wasted. A specific CPU isn't chosen because of PCIe lanes alone.

That’s true. I’m on the border between 9820x and 7820x. These two are pretty close performance wise. I was hoping that the higher PCIe lane count of 9820x would have allowed more direct connections for better overall performance of the system.