Xilinx Alveo X3522 User Manual page 40

Physical Medium
Latency will be affected by the type of physical medium used: twinaxial (direct-attach) or fiber.
This is because the physical media interface chip (PHY) used on the adapter can introduce
additional latency.
Modules Fitted
Latency can also be affected by the type of SFP/DSFP/QSFP module fitted.
PCI™ Express Lane Configurations
PCI Express (PCIe) interfaces can function at different speeds and widths. This is independent of
their physical slot size. The possible widths are multiples ×1, ×2, ×4, ×8, and ×16 lanes of (2.5
Gb/s for PCIe Gen 1, 5.0 Gb/s for PCIe Gen 2, 8.0 Gb/s for PCIe Gen 3 and 16.0 Gb/s for PCIe
Gen 4) in each direction.
On some server motherboards, choice of PCIe slot is important. This is because some slots
(including those that are physically ×8 or ×16 lanes) might only electrically support ×4 lanes.
X3522 adapters are designed for ×8 or ×16 lane operation. The X3522 driver will warn if it
detects that the adapter is plugged into a PCIe slot which electrically has fewer than ×8 lanes.
X3522 adapters require a PCIe Gen 4 ×8 or Gen 3 ×8 slot for optimal performance. The X3522
driver will warn if it detects that the adapter is placed in a sub-optimal slot.
Warning messages can be viewed in dmesg from /var/log/messages.
The lspci command can be used to discover the currently negotiated PCIe lane width and
speed:
$ sudo lspci -d 10ee: -vv
01:00.0 Ethernet controller: Xilinx Corporation Device 5084
Note: The supported speed might be returned as 'unknown', due to older lspci utilities not knowing how
to determine that a slot supports PCIe Gen. 3.0/8.0 Gb/s or PCIe Gen 4.0/16.0 Gb/s.
In addition, the latency of communications between the host CPUs, system memory and the
X3522 PCIe adapter might be PCIe slot dependent. Some slots might be "closer" to the CPU, and
therefore have lower latency and higher throughput. If possible, install the adapter in a slot which
is local to the desired NUMA node.
Please consult your server user guide for more information.
UG1523 (v1.0) October 18, 2022
Alveo X3522 User Guide
...
LnkCap: Port #0, Speed 8GT/s, Width x16, ASPM not supported
...
LnkSta: Speed 8GT/s (ok), Width x16 (ok)
Chapter 5: Tuning
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