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DrDave6

macrumors newbie
Original poster
Sep 14, 2010
4
0
Toronto, Canada
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

macrumors 65816
Oct 29, 2020
1,295
3,299
Buffalo, NY
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.
 
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