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[X570M Pro4] PCIe lanes

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newklear View Drop Down

Joined: 30 Jan 2020
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    Posted: 30 Jan 2020 at 12:40am
Hello and greetings to everyone since this is my first post here,

I am planning to build my new workstation around X570M Pro4 mobo and I would like to clarify the PCIe bandwidth and speeds.

The CPU will be Ryzen 9 3900x and I will have one SSD NVME M.2 drive. I am planning to have one RTX2070 super for start and then some time later add a second one on the second PCIe slot. As far as I know current graphics cards do not utilize the entire bandwidth of PCIe 4.0x16. PCIe 3.0 x8 is just enough for them. My question is: when I add the second GPU in a few months can I configure the PCIe slots in a way that none of the GPU will be bottlenecked? Are both PCIe slots equal in terms of bandwidth possibilities? Can they be configured to share bandwidth between both GPUs equally? I will use the GPUs for rendering, not for gaming.

Many thanks for clarification.
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Fred182 View Drop Down

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fred182 Quote  Post ReplyReply Direct Link To This Post Posted: 06 Feb 2020 at 6:31am
Good question. In that configuration it will operate as such:

AMD Ryzen series CPUs (Matisse)
- 2 x PCI Express 4.0 x16 Slots (PCIE1/PCIE3: single at x16 (PCIE1); dual at x16 (PCIE1) / x4 (PCIE3))
(Matisse is the codename for the 3000 series of Ryzen Desktop parts).

The 2070S does not support 4.0 so it will run at
x16 3.0 on the primary slot
x4 3.0 on the secondary slot.

How much this will impact you will vary greatly on your workload.
I know you're not gaming but or perspective: Nvidia does not allow SLI mode when the second slot is in x4 mode and the board itself isn't certified for SLI.
Some workloads are bandwidth-intensive and/or memory-intensive and those will be the most impacted workloads.
The easiest way to tell is to search for a Youtube video of 'how does PCIE bandwidth affect... Adobe Premier, 3DS Max, etc'.

You would have to step way up to get an 8x/8x (which would be enough for nearly any workload) configuration and there only appear to be ATX boards at this time that support that (someone else can certainly correct me if they find an MATX 8x/8x board, looking at Newegg there doesn't seem to be one.

To answer your final questions: no, you can't allocate bandwidth to suit your needs. On the board you reference it physically only as the connectors for a x4 connection on the second slot.

There is the concept of PCI Lane Bifurcation but A) it's a relatively advanced topic that exceeds a forum post and B) I would not use it to power graphics cards to to the power requirements (GPUs draw up to 75 watts through the motherboard. Having two GPUs share the same slot could destroy the slot and/or risers.) You might run into this topic if you search for it so that's why I'm stating it here.

I would disagree with 'current graphics cards do not utilize the entire bandwidth of PCIe 4.0x16. PCIe 3.0 x8'
They don't see much improvement (1-3%) for GAMING but AMD has shown substantial performance increase even at 16x 3.0 vs 16x 4.0. (Again, workload dependent, they showed... I want to say Blender?).

With that being said, I couldn't recommend this motherboard in this configuration for workstation use. If it were me I'd consider a single, higher-end GPU or a full ATX board that supports at least 8x.

Again, if your workload is entirely compute-limited and you're willing to roll the dice then buy your second card from somewhere with a good return policy.
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