expert parallelism

Active parameters describe one token. A decode step serves a batch, and the batch decides how many experts get pulled through HBM. The answer computes that number and shows where the MoE cost advantage actually comes from.

Every MoE layer sends each token out to its experts and gathers the results back, twice per layer, 58 times per step. The answer turns DeepSeek V3's config into bytes per token, splits them between NVLink and the NIC, and shows what node-aware dispatch saves.

Tensor parallelism splits a GQA cache by KV head. MLA has no KV heads to split, so TP copies the whole cache onto every GPU without any warning. The answer measures that waste on an H200 node and prices the fix.

Expert parallelism runs at the speed of the busiest GPU, and the router decides which GPU that is. The answer finds the batch size at which skew starts to cost compute, measures what placement and replication recover, and shows why a placement fit to yesterday's traffic decays.

Eight GPUs per NVLink domain shaped every serving layout of the Hopper era. Seventy-two moves the boundary that expert parallelism and KV transfer used to cross over the NIC. The answer prices what moves inside the rack, what still does not fit, and the failure mode that comes with it.

Collectives run at the speed of their slowest link, so one NIC that trained at a quarter of its rate slows every GPU in the instance by the same amount. The answer shows the arithmetic that makes a 4x slower link a 2x slower step, then bisects the cluster to the culprit.

Inside one batch there is nothing to overlap: every layer waits for its all-to-all. Split the batch in two and one half computes while the other communicates. The answer models the gain, finds the batch size where splitting makes things slower, and checks it against a shipped default.

A 1 TB FP8 model fits on one B200 node and needs four H100 nodes. That single fact sorts the two generations into roles. The answer assigns them, sizes both pools, and then deals with the precision mismatch that shows up when the two halves disagree about the weights.

Compare expert counts with rank-level completion times. Explain why the longest combine span can occur on a lightly loaded rank, then test a placement change against a communication control.