MP 7,1 PCI Express Terminology

[DrDave6]

I know that the 2023 Mac Pro has a bunch of PCI Express slots. However, I remember reading about one of the downsides of this box is that all the PCI Express slots share the same ??? on the bus. I’m not sure how to refer to this and what the issue is.

Can anyone clarify as I want to do more research on this.

Appreciate all feedback.

 

[bradman83]

The M2 ultra supports 32 lanes of PCIe 4.0. 8 of these lanes are dedicated to the SSD, the remaining 24 lanes are reserved for the PCIe slots.

The M2 Ultra Mac Pro has four PCIe 4.0 x8 and two PCIe 4.0 x16 slots. That adds up to 64 lanes vs the 24 lanes available. The M2 Ultra talks to the expansion slots via a PCIe switch chip that routes those lanes as needed based on what cards are actually in use, similar to how a network or HDMI switch operates.

So in theory if you had multiple cards being used simultaneously it could saturate all 24 PCIe lanes and result in reduced performance, but again that would require using multiple cards at full blast and having those cards use all of the lanes allotted for a given slot. Idle cards would not take up lane space.

 

[joevt]

The MacPro7,1 has four separate PCIe gen 3 x16 connections to the CPU: Slot 1, Slot 3, Pool A, and Pool B.
See page 11 at https://www.apple.com/mac-pro/pdf/Mac_Pro_White_Paper_Aug_2021.pdf
Pool A and Pool B are configurable. All Thunderbolt PCIe is on Pool B.
There are other threads on MacRumors that discuss the MacPro7,1, PCIe, and the Expansion Slot Utility.