stragglers

Every collective ends when the last rank arrives, so a single throttled GPU taxes 16,383 others. The arithmetic of that tax, the per-rank timing that finds the rank in one step, and the ranked list of causes from a hot GPU to a NIC with symbol errors, each with the command that confirms it.

A collective stops when one participant stops, and a job of a thousand ranks has no way to distinguish a link that will come back in two seconds from a rank that has died. What the job actually does during those seconds, the cost per flap, and why the fault passes every test you would run.

A training run has one number that matters and a handful that explain it. Goodput as the top-level metric, the per-rank timing that finds a straggler among a thousand, the hardware layer beneath it, and the three dashboards that serve three different people asking three different questions.

Five causes produce the same symptom and each has a check that takes under a minute. The order to run them in, what each one eliminates, and the arithmetic showing that one slow node in a thousand costs the whole job its speed rather than a thousandth of it.

Power rises faster than clock speed does, so giving up thirty percent of the power costs closer to eleven percent of the throughput. The relationship behind that, the uniformity requirement that matters more than the level, and why an uncoordinated cap is worse than a deeper coordinated one.