New Section 11 'TP vs CP: the shared-budget tradeoff' on the
Parallelism rules tab. Captures the core insight that fixing TP·CP
doesn't equalize the two configurations — larger TP additionally
divides model-weight traffic, larger CP only divides the sequence
dimension.
Five sub-tables:
11a. Same TP·CP = 16 → different weight sharding
(TP=8/CP=2 reads 4× fewer weight bytes per PE than
TP=2/CP=8 at the same KV division)
11b. What each dimension actually shards
(TP·CP·PP·DP × Weights·KV·Attention-math)
11c. When to prefer TP vs CP (6 situations mapped to the winning
axis with reasoning)
11d. CP communication profile (prefill vs decode) — bytes/hop,
overlap potential, real cost, notes on kv vs qoml variants
11e. Decode cost decomposition mental model:
T_decode = T_weight_read(TP) + T_KV_read(TP,CP)
+ T_TP_comm(TP) + T_CP_merge(CP)
Reframes the 'is CP heavier than TP' question as 'which cost
dominates T_decode, and which axis attacks it?'.
Smoke test asserts the section title appears exactly once.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
mukesh
bfe3e0e8d1