dcp: bulk-prefetch three shared CKV layers over copy-engine transport

[FujitsuPolycom]

Summary

Groups the three eligible GLM Shared-layer selected-record CKV prefetches into
one B12X copy-engine exchange and one arrival barrier.

• Reuses the Full layer's finalized per-sequence MTP union/remap.
• Reuses existing sparse payload workspaces; no second workspace slab.
• Shares one completion event across S1/S2/S3 and waits once before S1.
• Keeps direct transport, partial groups, older B12X, and capacity overflow on
  the existing per-layer path.
• Adds strict auto|ce|direct transport selection and a default-off bulk flag.
• Preserves C3-C8 fallback behavior.

Stacked Sparkinfer dependency: FujitsuPolycom/sparkinfer#3.

The final stack is rebased on local-inference-lab/vllm@73e4a8cde and
local-inference-lab/sparkinfer@ec2cd4d. The package rename uses the native
sparkinfer.comm.pcie APIs; no retired b12x.distributed shim is included.

Configuration Tested

• ai01: 4x RTX PRO 6000 Blackwell, 96 GiB each
• TP4 / DCP4 / MTP3
• max model length 300,000; reported KV 302,047 tokens
• batch 2,048; graph 32; max sequences 8
• nvfp4_ds_mla, native 432-byte records, BF16 RoPE
• replicated indexer; depth-3 full CKV prefetch
• selected-record sparse decode over CE; NCCL TP all-reduce; no LMCache

The matched A/B changed only
VLLM_B12X_MLA_SPARSE_DECODE_BULK_PREFETCH=0|1.

Performance

Context	Conc.	Per-layer CE	Bulk CE	Delta
8K	1	70.20	77.05	+9.76%
8K	2	90.96	100.23	+10.19%
32K	1	72.00	74.80	+3.89%
32K	2	90.81	97.35	+7.20%
64K	1	70.86	79.55	+12.27%
64K	2	88.30	98.33	+11.36%
128K	1	66.10	71.50	+8.17%
256K	1	59.80	65.36	+9.29%

• Comparable 64K+ geometric mean: +8.50%
• Coding median: 99.63 -> 103.79 tok/s (+4.17%)
• Every comparable cell remained above the frozen 97% floor.
• C4/C8 fallback remained within noise.
• KV capacity remained 302,047 tokens in both arms.

Correctness And Tests

• Image build: 117 passed, 1 skipped.
• B12X four-GPU eager/graph/mixed-lifecycle gate: 1 passed; DCP8 skipped on
  the four-GPU host.
• Deterministic model outputs matched twice at 8,095, 64,032, and 127,998
  prompt tokens.
• C2 bulk and C4 fallback model smoke completed successfully.
• Bulk mode is default-off and rejects invalid flag values instead of silently
  disabling the experiment.

Historical matched-A/B image:
ai01/glm52-fork-pr4:bulk-ce-final
(sha256:9d20416c82be7c398b1f833f3d51b762c3fe3f73fc57a05f88312ca361a4a6ac).

Rebased validation image: ai01/glm52-sparkinfer-rebase:cpu-validated.
It passed 56 Sparkinfer tests, 120 vLLM policy/workspace tests, the
four-GPU direct/CE transport gate (3 passed, 1 DCP8 skip), retained
302,047 KV tokens, and returned the expected deterministic API output.
Its short rebase sanity run measured 3,437 / 3,165 prefill tok/s and
86.3 / 85.1 C1 decode tok/s at 8K / 64K.

AI Assistance

Implementation, tests, benchmarking utilities, and documentation were
developed with OpenAI Codex. Results were manually reviewed and validated on
the target four-GPU host. The scheduling design is informed by koush's sparse
CKV prefetch work and the existing B12X/v18 GLM stack.

Design document

GLM-5.2 Sparse CKV under Decode Context Parallelism is the canonical architecture, configuration, validation, and upstream-decomposition reference for this PR stack.

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