The bs=1 MLA/index prefetchers replaced their consume-side trailing-range
NCCL all-reduce with the staging exchange: fill current rows straight into
this round's staging span (token-KV collapses to one cached index_copy;
the index fill kernel is just pointed at the staging page inverse),
cp_symm_barrier, then gather ALL current pages — this rank's own included
— from the stagings into the prefetched dense buffer. The symm+prefetcher
FAIL_FAST is gone.
Rank-uniformity moves with it: staging registration now also happens in
maybe_create (batch-logical gates, before any per-rank miss can diverge),
because with a prefetcher active the sync compose runs only on per-rank
misses and its lazy collective registration would hang. A hit/miss
divergence itself stays barrier-safe — both the prefetch consume and the
sync-compose fallback execute exactly one begin_round + barrier per
(layer, kind), and the counting barrier is shape-free (unlike the AR pair
it replaces, which would shape-mismatch).
Found by the new index test phase: the fill/remap kernel family skips
page id 0 as the SGLang dummy page, so a 0-based first staging slot was
never written. The staging layout now reserves row 0 (slot of current
page i = i + 1) for every kind, matching the convention instead of
depending on per-kernel behavior.
Launch-path cost: per-(kind,parity) peer pointer tables and the
[pool|staging] concatenations are precomputed/cached (identity pinned by
holding the pool-table reference); all prefetch descriptors, staging row
indices, and mixed_locs are built once per batch.
Validated on g0033 8xH200: 151 unit tests; 8-rank byte-exactness for
token sync symm (8 layers), index sync symm (4 layers, new phase), and
MLA + index prefetch consume_prefix_with_current vs the legacy sync
compose.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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