CN - ASUS AMD AM4 Pro WS X570-Ace ATX Workstation Mainboard

[nabsltd]

worth noting is that the 4th slot is PCIe4x8 and then bottlenecked by the chipset PCIe4x4 link, so if you have a PCIe3x8 card you will get the full bandwidth

That's not how PCIe negotiation works.

If the slot has been dropped to 4 lanes because of a PCIe switch forcing the lanes somewhere else, then you will only get 4 lanes to the slot. After that, the slot and card negotiate the version, so since the card in your example is PCIe 3, that's the link speed.

So, the result would be PCIe 3.0x4.

 

[UhClem]

That's not how PCIe negotiation works.

If the slot has been dropped to 4 lanes because of a PCIe switch forcing the lanes somewhere else, then you will only get 4 lanes to the slot. After that, the slot and card negotiate the version, so since the card in your example is PCIe 3, that's the link speed.

So, the result would be PCIe 3.0x4.

No.
@thulle was correct!

 

[thulle]

So, the result would be PCIe 3.0x4.

So then please, enlighten me with your wisdom, how am I able to pull 50+ gbit/s over that PCIe3x4 link?

 

[luckylinux]

So then please, enlighten me with your wisdom, how am I able to pull 50+ gbit/s over that PCIe3x4 link?

I'm not taking anybody's Side, but how are you doing your Test ?

Are you Looping from one NIC Port to the Other ? If so, unless the Interfaces are within different Network Namespaces, everything will be bypassed through the Kernel since it is aware that both are "Local" Interfaces.

If you are doing from one NIC Port to a different PC, disregard my Message, then you are obviously right.

It could just be that the upstream is PCIe 4.0 x4, but the downstream is PCIe 3.0 x8, but I'm not a PCIe Switch Expert so I am not sure whether that is possible or not.

EDIT 1: and of course, if you are doing e.g. iperf3, make sure that the IP you are testing against is REMOTE. I also had crazy Results when I set the server IP to the Local IP Address (I was using a different Port and iperf3 Systemd Service was apparently configured to start automatically after Boot).

 

[thulle]

I'm not taking anybody's Side, but how are you doing your Test ?

iperf from a remote server.....

 

[nabsltd]

So then please, enlighten me with your wisdom, how am I able to pull 50+ gbit/s over that PCIe3x4 link?

It would appear there is a full PCIe switch chip in the system that converts the PCIe 3.0x8 into PCIe 4.0x4. Most PCIe switches just route lanes and don't perform a full data unpack and repack.

 

[grenskul]

AMD chipsets switch they don't negotiate (and so do all modern chipsets I think) .
Otherwise it would be impossible to have that many downstream devices.
It has 4x pcie 4 of bandwidth and it switches that to whatever it needs.
It can switch a 4x pcie 4 link to an 8x pcie Gen 3 link. But yea it's shared with everything else so no full performance obviously.