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Even after reading several related questions and answers I'm still not sure if I got it right so I'm asking this just to make sure I am.

I'd like to build s PC based on Asus WS Z390 PRO with 4 GPUs and an i3 9100 CPU. My dilemma is if I will get all 4 PCIe slots to provide 8x.

Regarding MBO specs (https://www.asus.com/us/Commercial-Servers-Workstations/WS-Z390-PRO/specifications/) it supports 8x/8x/8x/8x combination, but I'm not sure if a CPU's 16 PCIe lanes (https://ark.intel.com/content/www/us/en/ark/products/134870/intel-core-i3-9100-processor-6m-cache-up-to-4-20-ghz.html) will be a bottleneck or not ... As far as I've understood CPU PCIe lanes are addition to MBO PCIe lanes and should not prevent me from achieving 8x/8x/8x/8x performance.

Thanks.

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  • If the motherboard supports 8x/8x/8x/8x it doesn’t matter if the processor has 16 lanes it cannot possibly be a bottleneck. Since there isn’t any other choice other than a 9th Generation Core processor for that motherboard (assuming you want that motherboard).
    – Ramhound
    Commented Apr 13, 2021 at 11:35
  • The simple fact seems to be that no matter what processor you choose it will only have 16 lanes, as seen by searching other processors on the Intel site. That basically means that for an x8/x8 configuration then most likely the processor is servicing two slots, with the other two coming from the PCH, both allocating at most 16 lanes each to the x16 lanes for a total of 32 lanes in 4 slots and resulting in the x8/x8/x8/x8 configuration. That would agree with the x16/x8/x8 as well as either the PCH or CPU would have to service (and split between) two slots while the other handles a single slot.
    – Mokubai ♦
    Commented Apr 13, 2021 at 11:44

1 Answer 1

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It is clearly stated on their specifications page:

Expansion Slots

4 x PCIe 3.0 x16 (single at x16, dual at x16/x16 mode, triple at x16/x8/x8 mode or quad x8/x8/x8/x8 mode)
1 x PCIe x4 (max at x4 mode)

What PCIe ports are connected to the CPU and which are connected via the PCH to a PCIe bridge is down to the manufacturer. Given the four slots I would expect 2 to connect to the CPU and two to the PCH. The motherboard manual may tell you which slots to use first to achieve a particular configuration.

Even an i7-9700 has only 16 PCIe lanes, so if you are going to be limited with your i3 (which might have other CPU performance related limits), then you will be limited with a better processor as well.

i3-9100 vs i7-9700 comparison

enter image description here

On top of that your PCH has a high speed bus to the CPU, and then splits out a further 24 PCIe lanes, 16 of which are apparently shared to the two x16 slots not services by the CPU.

Intel Z390 Specifications

enter image description here

It claims here that it only supports up to x4, but that may depend on specific parts and configuration. Asus might have a version or configuration that supports x16 and x8/x8, or might be using port multiplexers and buffers to "share" more ports, the Intel documentation will be for direct connection and Asus are likely using a non-standard setup.

This is the Layout you'll have:

https://www.intel.co.uk/content/dam/www/public/us/en/images/diagrams/16x9/z390-chipset-block-diagram-16x9.png

Note the 16 lanes direct from the CPU (top left), and the 24 lanes from the PCH (middle left).

Found via Google Images and apparently from https://ark.intel.com/content/www/us/en/ark/products/133293/intel-z390-chipset.html (maybe one of the spec sheets)

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  • Thank you for your answer but you kind of missed my point ... I'm just worried that CPU 16 PCIe lanes might not be enough to support MBO's 8x/8x/8x/8x configuration, but it seem like it shouldn't be an issue ... You comment above clarifies a situation a bit more ... CPU will provide 2 x8 lanes and other 2 x8 will be provided by PCH. Therefore this combo should work without any problems.
    – h45h3ncryp73d
    Commented Apr 13, 2021 at 11:49
  • Your CPU might only have 16 lanes, but your PCH (Platform Controller Hub - the chip that the CPU uses to talk to other devices) will be servicing the other lanes and slots. Your PCH has another 24 lanes available: ark.intel.com/content/www/us/en/ark/products/133293/…
    – Mokubai ♦
    Commented Apr 13, 2021 at 11:52
  • @h45h3ncryp73d added some more details.
    – Mokubai ♦
    Commented Apr 13, 2021 at 12:08

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