Aegis-GPU4.4 Signal quality on cables
Counters, BER and eye opening with mlxlink, and fabric-wide error checks.
Key points
BER (bit error rate) is the share of bits received wrong. The eye opening measures how clean the signal is. A wider eye means a cleaner signal. mlxlink checks and debugs link status. -c shows counters and BER. -e shows the eye.
What NVIDIA says (3)
“The mlxlink tool is used to check and debug link status and related issues.”
“-c |--show_counters Show Physical Counters and BER Info”
“-e |--show_eye Show Eye Opening Info”
Error counters count events such as symbol errors on each port. Rising counters point to a bad cable or module. ibqueryerrors checks every port in the fabric.
What NVIDIA says (1)
“The default behavior is to report the port error counters which exceed a threshold for each port in the fabric. The default threshold is zero (0).”
ibdiagnet scans the fabric with directed route packets. Directed route means the packet follows a path given hop by hop, so it works even before routing is set up. Its stages include error counter checks and link width and speed checks. A link at lower width or speed than planned is a cabling problem.
What NVIDIA says (2)
“Scans the fabric using directed route packets and extracts all the available information regarding its connectivity and devices.”
“Link width and speed checks”
Key terms
- mlxlink: The NVIDIA tool that checks link state, cable or module details, error counters and the signal eye on a port.
- Bit error rate: The share of bits received wrongly on a link; mlxlink shows it with the physical counters.
- ibdiagnet: An InfiniBand tool that scans the whole fabric and reports connectivity, devices and link width and speed.
Try it
Sample question
You suspect a marginal InfiniBand link. Which mlxlink options show its signal quality?
Show the answer
Answer: -c for physical counters and BER, and -e for eye opening.
BER (bit error rate) is the share of bits received wrong. The eye opening measures how clean the signal is. A wider eye means a cleaner signal. mlxlink checks and debugs link status. -c shows counters and BER. -e shows the eye.
What NVIDIA says (3)
“The mlxlink tool is used to check and debug link status and related issues.”
“-c |--show_counters Show Physical Counters and BER Info”
“-e |--show_eye Show Eye Opening Info”
Practice 4.4 (3 questions) Full Cluster Test and Verification guide
← 4.3 Single-node NCCL and NVLink Switch · 4.5 Confirming cabling →