One Query, Four GPUs: Tracing a Distributed Training Stall Across Nodes

TL;DR A single straggling node held up a 4-node distributed training job. We found it by fanning out one SQL query to all four nodes and getting the answer in under a second. This is distributed GPU training debugging with eBPF – no central service, no Prometheus, no time-series database, just the same single-binary agent already running on each machine.