[Feature][SimpleCPU] Load fine-grained hybrid prefix hits - #54736
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Included review availability: Your plan provides up to 10 included reviews per hour; 7 remain after this review. WalkthroughThe scheduler now supports hybrid attention–Mamba cache groups, DCP-aware sizing, fine-grained external hits, boundary handoffs, partial full-attention tail stores, admission statistics, and cache-mode-specific positional handling. Tests cover these flows. ChangesHybrid KV offload
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to Eager hybrid offload may store a completed full-attention block twice, wasting CPU cache capacity and producing duplicate transfers. The duplicate-selection path should be guarded or conclusively ruled out before merge. Sequence Diagram(s)sequenceDiagram
participant Scheduler
participant SimpleCPUOffloadScheduler
participant KVCacheCoordinator
participant CPUBlockPool
Scheduler->>SimpleCPUOffloadScheduler: request external CPU hit
SimpleCPUOffloadScheduler->>KVCacheCoordinator: read group block sizes
SimpleCPUOffloadScheduler->>SimpleCPUOffloadScheduler: validate scheduler-block alignment
SimpleCPUOffloadScheduler->>CPUBlockPool: place loaded attention and Mamba blocks
CPUBlockPool-->>Scheduler: return loaded KV blocks
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In `@tests/v1/simple_kv_offload/test_scheduler.py`:
- Around line 2367-2373: Update the logging assertion around
SimpleCPUOffloadScheduler.get_num_new_matched_tokens to use the caplog_vllm
fixture instead of caplog, while preserving the WARNING level and existing
message assertion.
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Use the coordinator's resolved per-group block sizes when mapping SimpleCPU offload transfers in hybrid DCP layouts. Only full attention is sharded across DCP ranks; mamba, sliding window and chunked-local specs keep replicated per-rank state and must not be scaled by the DCP world size. Scaling every group by cp_world_size mapped the replicated groups onto the wrong block geometry. Expose group_block_sizes on the coordinator so the resolved sizes have a single source, and apply it to the lazy watermark estimate too, where the same hand-rolled formula under-counted the blocks to reserve for sliding window groups. Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
Enable eager SimpleCPU offload to consume hash-aligned hybrid prefix hits after a scheduler-block-aligned local prefix. The load path maps accepted token ranges through each cache group's resolved physical block size, so partial external suffixes land in the correct destination blocks. The per-group block count now rounds up, because a fine-grained hit is aligned to the hash block size rather than to every group's block size; hash alignment is still asserted, since it is what lets the external suffix start on a boundary every group can address. Transfers are whole-block copies, so an external suffix cannot start inside a destination block. A request whose local prefix is not scheduler-block aligned keeps its local hit and skips the external lookup. Store exact boundary handoffs before positional scans, and avoid positional scans for Mamba align tables because those block tables can move or null interior state. Mamba all-mode remains eligible for positional stores. Boundary stores extend the kv-cache-event metadata alongside the block ids, so the two lists stay index-parallel for the completion path. Handoffs are dropped for requests that finished or were preempted in the same step. The scheduler drains them without filtering by request liveness, and a departing request's blocks may already back another request, so reading them would publish unrelated KV under a valid hash. This mirrors the conservative drop the mooncake store performs. The local prefix length resolved by get_num_new_matched_tokens() is carried through to the load path rather than re-derived by counting hashed full-attention blocks, which assumed every hashed block in the group's table belongs to the contiguous local prefix. The offered boundary is not used to derive a cache key here, so no alignment filter is needed: the key comes from the handed-off block itself. Lazy offload remains scheduler-block aligned by disabling partial hash hits in the CPU coordinator. Only the hybrid coordinator accepts that flag. Every branch that declines a hand-off is silent by design, so each decline reason is counted and exposed through get_boundary_store_stats(). Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
Kimi-K3 DCP8 can finish a prompt at a hash boundary that is not a full-attention effective-block boundary. Mamba align hands off the prompt-tail state at that fine boundary, while the positional scan only preserves whole attention blocks. Hybrid lookup requires every group to hit the same boundary, so the CPU coordinator would otherwise see attention at the previous full block and Mamba at the prompt tail, then reconcile to zero. Queue a SimpleCPU-only finish-time store for that attention block when fine-grained hits are enabled. This mirrors FullAttentionManager._cache_partial_tail_block: only the final prompt hash boundary is eligible, and boundaries landing on a physical block edge are already covered by the positional scan. Attention block tables are append-only, so the block is located positionally, as the mooncake store and offloading connector also do; no new core hand-off is required. The boundary key is registered on the source block at completion along with its other hashes, so nothing has to be captured at queue time. Decode tokens can fill the boundary block before the request finishes, in which case the positional scan already selected it; skip the tail store then so one GPU block never consumes two CPU blocks. The partial-tail store is best effort when the CPU block pool is full. Dropping it is a cache miss, not a correctness failure. Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
A request can contribute both positional attention blocks and explicit Mamba boundary blocks to one store event. Keep all selected blocks, but report the request once so completion bookkeeping has a canonical event-to-request mapping. Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
Record event metadata only for concrete block hashes. Store admission guarantees the primary hash exists, while sparse secondary hashes may be absent and must not become metadata keys. Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
Mamba align groups use explicit boundary handoffs and are excluded from positional eager scans. Without a handoff, assert that only full-attention event metadata is emitted. Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
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…ct#54736) Signed-off-by: Yichao Zhu <Yichao.Zhu@amd.com>
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Purpose
On hybrid models, SimpleCPU offload cannot serve a fine-grained external prefix hit reliably:
alignmode nulls interior states and relocates speculative blocks in place, so its block table cannot be resolved by a monotonically advancing positional cursor. The connector must consume the exact boundary handoffs already published by the KV cache manager.Stacked on #54735.
Changes
allmode and the other append-only group types keep positional scanning.boundary_state_offloadschannel.Runtime Validation
All results below use source head
9cf76fe57with eager SimpleCPU offload and the defaultprefix_cache_retention_interval=0.Kimi-K3 TP8/DCP8
Deterministic dummy cold/hot/pressure/replay with 32 GPU blocks:
Real-weight full GSM8K, 20-shot, 1319 samples, concurrency 64, 512 GPU blocks, workload-driven eviction without a cache reset between rounds:
Real-weight AgentX/AIPerf fast, 15-minute profile, concurrency 16, 512 GPU blocks:
The constrained GPU block pool intentionally forces local eviction. This validates the external offload path and is not presented as the normal unconstrained cache distribution.
Qwen3.5-4B TP8/DCP2
The Qwen runs use 8x B200 with CUDA
FLASH_ATTN, which provides the decode LSE required by DCP for ordinary GQA. This covers a second hybrid architecture whose recurrent groups are GDN-backedMambaSpecgroups.Deterministic dummy cold/hot/pressure/replay with 32 GPU blocks:
Real BF16, 20-shot GSM8K N20, concurrency 1, 32 GPU blocks:
The Qwen score is reported only as a replay consistency check for this small model and sample; the relevant correctness signal is unchanged output and accuracy after external-cache replay.
Limitation
Transfers remain whole-block copies. If a local prefix is not scheduler-block aligned, SimpleCPU keeps the local hit and skips the external lookup rather than starting a transfer inside a destination block.