On 9/1/2022 10:06 AM, Skybuck Flying wrote:
> The information website about: MEG-X670E-GODLIKE AM5 socket motherboard claims to reach 128 gigabyte/sec for what they call lighting PCIe 5.0 ?!
>
> https://www.msi.com/Motherboard/MEG-X670E-GODLIKE
>
> How is this possible since PCIe 5.0 only reaches 64 GB/sec according to wikipedia ?
>
> https://en.wikipedia.org/wiki/PCI_Express
>
> So is one of them wrong ?
>
> Or is this something special ?
>
> Bye,
> Skybuck.
>

"Three PCIe Gen 5.0 x16 (x16/x8/x4 electrical)
that run at either x16/x0/x4 or x8/x8/x4 modes"

I think someone saw two PCIe Rev.5 slots and
assumed each carried 64GB/sec, which they don't.

Between the two main slots, there is only 64GB/sec
to work with.

A typical trick in the industry, is to include both
the TX wires and RX wires (slots are full-duplex),
and call this 128GB/sec, when in any reasonable use-case,
the data flow is mostly in one direction, and almost
zero in the other direction. Like, in a game situation,
the card mostly "gobbles textures", it does not
"spew them back". Maybe with SLI, there could be
some transfer in the secondary direction.

Since the "transfer target" for a slot, is system memory,
at some point on a motherboard design, the slot simply cannot
achieve the entire wire rate. System memory is half duplex
and either reads or writes. A typical computer memory
bus design, does not reach 100% efficiency, does not
transfer data on every available cycle. You can see a measured
value for what a system will do, when using memtest.
And 64GB/sec is a *lot* for the system memory bus
to handle. None of my systems here could handle that.

Sure, many wires in the computer have their "wire rate",
but as the rates keep rising, the opportunity to sustain
the rate is not there. An NVMe for example, the SLI cache
will run out after a while, and writes will slow down.
A device won't stay at 14000MB/sec forever.

Paul