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1. IL-0476 PCIe downgrade on one machine slowed a 128-machine V100 training job for 40 minutes through a PFC surge

   ByteDance2025

   Symptoms

   A distributed training task on 128 machines in ByteDance's production environment slowed down severely for 40 minutes but kept running, holding thousands of V100 GPUs at reduced speed. The alerting in place notified engineers only when a task stopped completely, so the slowdown itself raised no alert.

   Throughput degradation

   Trigger

   The PCIe link of one machine degraded from 6.4Gbps to 4Gbps during the training task.

   Root cause

   Hardware failure

   Component

   PCIe linkInfiniBand fabric

   Contributing factor

   No alert on limit

   Detection

   No alert fired because alerts came only when a task stopped. Engineers from several teams found the faulty machine by manual diagnosis, which took 40 minutes.

   Action items

   • Detect faulty machines automatically during training by comparing each machine's per-second monitoring metrics with the other machines (Minder)

   Lessons

   Some faults slow a task without stopping it, and alerting that waits for a halted task misses them. Logs alone are incomplete because they do not contain monitoring metrics such as PFC packet rates, and no single metric reveals every fault type, so ByteDance compares machines with each other on several metrics and treats a difference that lasts for minutes as a fault.

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