Addresses the opus review of the Step A/B compose stack:
- FAIL FAST on symm + prefetcher coexistence: the MLA/index prefetchers
issue their own per-span collectives and bypass the symm exchange, so
letting them coexist would make COMPOSE_SYMM a silent no-op on every
prefetch hit (measured "parity" that never ran). maybe_build_current_
page_writer_ranks now raises when a prefetcher is attached or
SGLANG_CP_SHARED_KV_ENABLE_MLA_PREFETCH is set.
- Arena parity now derives from a monotonic round epoch instead of raw
layer_id: EAGLE draft layers reuse decoder layer ids, so id-parity would
stop alternating halves (draft L0 -> next forward target L0 lands on the
same half) and repeated-id rounds would keep appending into one half.
begin_round(layer_id, kind) starts a new round (other half, offsets
reset) when the id changes OR a kind repeats; regression test included.
- Per-batch writer-ranks cache on the forward batch and a presence flag in
the plan key: previously the ~bs x current-pages writer list was rebuilt
AND tuple-hashed on every layer per call site, pure launch-path waste.
- Rank-uniformity invariant documented at _symm_exchange_current_pages
(any per-rank gate must go through a group agreement first) and an
actionable arena-overflow message naming SGLANG_CP_SHARED_KV_SYMM_HEAP_MB.
Deferred follow-ups (documented in the design doc): factor the two near-
clone v2 helpers, single barrier per F-layer (~308us/batch), persistent
device-side peer-ptr tables for the gather.
182 targeted tests + 8-rank GPU byte-exactness (v2 and symm) re-validated
on g0034 syh-dev-new.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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