Function marconi_snapshot_slots 

pub fn marconi_snapshot_slots(
    requested: usize,
    prefix_caching_active: bool,
) -> MarconiSlotDecision

Number of Marconi SSM-snapshot slots to RESERVE and ALLOCATE.

Two call sites MUST agree on this number, exactly as they must for the decode-rollback ring above, or a serve either under-reserves (runtime CUDA alloc failure after weights load) or over-reserves (preflight refuses a configuration the runtime could fund):

• spark-server preflight_reserve — sizes the pre-load GPU reserve;
• TransformerModel::new (impl_a1.rs) — allocates SsmSnapshotPool.

WHY a gate exists. The Marconi region’s ONLY consumer is the prefix cache: a slot is written by prefill_b_save_checkpoint / insert_*_snapshot and can only ever be READ BACK through a prefix-cache lookup that returns an ssm_snapshot id (prefix_cache.rs, “SSM state snapshot ID at the deepest matched node (Marconi caching)”). Without --enable-prefix-caching, build_prefix_cache installs NoPrefixCaching — no radix tree exists, no lookup can ever produce a snapshot id, and every reserved slot is unreachable for the life of the process. Yet --ssm-cache-slots defaults to 16 and was sized independently of the flag, so a serve with prefix caching disabled still reserved 16 × num_ssm_layers × (h_state + conv_state) bytes that nothing can restore from.

Measured on GLM-5.3-Flash NVFP4, 2× GB10, TP=2 EP=2, K=3, batch 1, GMU 0.90: 2380 MiB per rank — 16 slots × 34 KDA layers × (h 4.000 MiB + conv 0.375 MiB). Both widths are FP32 by construction (ModelConfig::ssm_h_state_bytes / ssm_conv_state_bytes each end in * 4), and --ssm-h-dtype f16-pool is opt-in, so the FP32 figure is what an ordinary serve reserves AND allocates: SsmStatePool reads the same two accessors (ssm_pool.rs:182), so reserve and residency agree. Confirmed by a paired A/B, same session, 90 s apart, identical flags (2 131072 1 0.90): post-load requirement 13.58 → 11.25 GB, a 2.33 GB drop that matches 2380 MiB exactly. The gated default now needs precisely what the same image required only when an operator passed --ssm-cache-slots 0 by hand (ANOMALIES A68).

🪤 GLM53-MEMORY-LEDGER-20260830.md §2/§4 records this region as “16 slots × 74.4 MB = 1190 MB”. That is the FP16-width arithmetic (h 2.000 + conv 0.1875 MiB/layer) and is exactly half; the same halving applies to its “SSM live state pool 1 slot × 34 layers = 74 MB” row. Trust the FP32 figure — it is what the code allocates and what the live A/B measured.

This is the same defect class the decode ring above already fixed: a pool reserved unconditionally while nothing could reach it.

Nothing degrades when the slots are dropped. prefill_b_save_checkpoint early-returns on !ssm_snapshots.is_enabled(), so there is no work and no warning spam on the prefill path; the only user-visible difference is that prefix-cache hits would recompute SSM state — and with the cache inactive there are no hits.

Env contract (read HERE and nowhere else):

• ATLAS_SSM_MARCONI_FULL (PRESENCE, house convention — =0 is NOT “off”): restore the old unconditional reservation. Accounting-safe over-reserve; the kill switch for this diet.