[Model Runner V2] Automatic prefix caching for GDN hybrid models (mamba_cache_mode=all) with MTP speculative decoding by anuragdutt · Pull Request #54637 · vllm-project/vllm

anuragdutt · 2026-08-31T21:53:12Z

Purpose

Automatic prefix caching for GDN hybrid models (mamba_cache_mode="all"), working under MTP speculative decoding, targeting Model Runner V2 (the default runner). A block is cached by checkpointing the GDN recurrent (SSM) state at mamba-block granularity during prefill and reusing it on prefix hits, instead of recomputing the whole prefix.

The stack:

Engine core (shared): kernel-chunk-aligned prefill clipping and block-aligned split in the scheduler; Mamba-group exemption from the EAGLE/MTP flag-all draft-group fallback; dense checkpoint-retention defaults for mamba cache modes; platform-level block-size validation.
Kernels/layers: per-chunk intermediate-state export from the FLA chunk kernel; block-aligned checkpoint scatter (gdn_scatter_block_checkpoints); dual read/write state anchors in the decode kernels (fused_recurrent, fused_sigmoid_gating, causal_conv1d) so a spec-decode step can read the previous step's running state while writing to the current anchor; the GDN metadata builder's all-mode anchor fields.
Model Runner V2 plumbing (new): per-request tracking of the previous step's write anchor, GPU-resident in MambaHybridModelState (one fused Triton kernel in preprocess_state, consume-once staging into prepare_attn), handed to the Mamba2/GDN builders via MambaHybridAttnMetadata.prev_last_scheduled_idx. Without it, all-mode + spec decode on V2 silently reads a stale SSM checkpoint whenever a draft rejection crosses a mamba-block boundary. The V1 runner gets the same anchor by widening its existing Mamba2-only gate to include the GDN builder (8 lines).

Relationship to existing PRs

#50172 (same author) is the V1-runner-era draft of this work. This PR re-bases it onto current main and adds the Model-Runner-V2-native plumbing, which #50172 predates; V2 is now the default runner, so this is the PR to review. A follow-up PR (#54612) with decode-path performance optimizations stacks on top. No third-party PR implements mamba all-mode prefix caching (checked open PRs for mamba prefix caching / GDN).

Correctness

Cold/warm byte-identity probes (B200, Qwen3-Next-80B-A3B-Instruct-NVFP4, greedy, ~8.9k-token prompt sent twice; server logs confirm Using V2 Model Runner):

config	warm `cached_tokens`	outputs byte-identical
TP1 all + MTP k=3	6912 / 8894	yes
TP1 align + MTP k=3	6720 / 8894	yes
TP4+EP all + MTP k=3	8064 / 8894	yes
TP4+EP align + MTP k=3	7840 / 8894	yes

All four configurations produce the same completion hash — SSM-state reuse is lossless under speculative decoding.

Unit tests (run on B200, CUDA 12.9 container):

pytest tests/v1/worker/test_mamba_hybrid_prev_anchor.py \
       tests/kernels/mamba/test_gdn_scatter.py \
       tests/kernels/mamba/test_fla_chunk_block_states.py \
       tests/kernels/mamba/test_fused_sigmoid_gating_dual_index.py \
       tests/v1/attention/test_gdn_metadata_builder.py

plus the shared-core suites added by this PR (tests/v1/core/test_mamba_all_mode_split.py, test_mamba_block_size_validation.py, test_mamba_chunk_size.py, prefix-caching tests) and the all-mode-vs-align parity kernels tests (test_gdn_all_mode_{prefill,decode,spec_decode}.py).

Performance

All-vs-align throughput Pareto on the same build (B200, NVFP4, MTP k=3, aiperf, 240 requests/cell, OSL 1024, prefix cache reset per cell — i.e. a cold-cache workload showing all-mode's overhead side; the benefit side is the cached-token reuse above):

The all-mode checkpoint-write overhead vs the align reference is single-digit to low-teens percent across the ladder; the follow-up optimization PR (#54612) addresses it.

Disclosure

AI assistance (Claude Code) was used to develop and test this change. The submitting human has reviewed every changed line and will run/re-run any requested validation.

Scheduler support for all-mode (kernel-chunk-aligned prefill clipping and block-aligned split), Mamba-group exemption from the EAGLE/MTP flag-all draft-group fallback, dense checkpoint retention defaults for mamba cache modes, GDN chunk-size derivation, and platform-level all-mode block-size validation. Signed-off-by: Anurag Dutt <andutt@nvidia.com>

Per-chunk SSM checkpoint export from the FLA chunk kernel, block-aligned checkpoint scatter, dual read/write state anchors for decode and spec decode (in-kernel index derivation from the block table in fused_recurrent / fused_sigmoid_gating / causal_conv1d), the GDN metadata-builder all-mode anchor fields, and the Qwen3-Next layer all-mode forward paths. Includes the convolution-window fix (true spec query length as max_query_len) and the gating index fix (in-kernel state-slot derivation). Signed-off-by: Anurag Dutt <andutt@nvidia.com>

Signed-off-by: Anurag Dutt <andutt@nvidia.com>

…c decode The GDN/Mamba2 builders' spec-decode read anchor (prev_last_scheduled_idx) was fed only by the V1 runner; on the V2 model runner every step took the last-computed-block fallback, which is valid only for first-step or untracked requests, giving a stale state read after a draft rejection across a mamba-block boundary. Track the anchor GPU-resident in MambaHybridModelState, mirroring the V1 postprocess_mamba_all / preprocess_mamba_all_specdec pair: a fused kernel in preprocess_state stages the previous step's anchors in batch order and records this step's anchor for full-decode-window rows, and prepare_attn hands the staged values to the Mamba2/GDN builders via MambaHybridAttnMetadata. Signed-off-by: Anurag Dutt <andutt@nvidia.com>

…ff format Signed-off-by: Anurag Dutt <andutt@nvidia.com>

Dummy runs skip preprocess_state but still build attention metadata, so without a validity marker they would consume the previous real batch's staged anchors in the wrong row order (contained today only by the zeroed dummy block tables). Serve the staged values at most once per staging; batches that did not run preprocess_state get the all-untracked (-1) fallback. Signed-off-by: Anurag Dutt <andutt@nvidia.com>

The upstream V1 runner passes the previous-step write anchor only to Mamba2AttentionMetadataBuilder; GDN all-mode with speculative decoding takes the last-computed-block fallback on every step, reading a stale SSM checkpoint after a draft rejection crosses a mamba-block boundary. Widen the isinstance gate to include GDNAttentionMetadataBuilder (the builder already accepts and resolves the kwarg). Signed-off-by: Anurag Dutt <andutt@nvidia.com>

github-actions · 2026-08-31T21:53:37Z

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njhill · 2026-08-31T22:37:09Z

Thanks @anuragdutt. We are thinking of removing "all" mode altogether soon though.

djmmoss · 2026-09-01T02:50:07Z

Hi @njhill , what's the reason for remove "all" mode? lack of support for the feature or something else?

claude

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

Copilot

Pull request overview

This PR enables automatic prefix caching for GDN hybrid (gated-delta / FLA) models when mamba_cache_mode="all", including correct behavior under MTP speculative decoding, targeting the default Model Runner V2. It adds per-block SSM checkpointing at mamba-block granularity during prefill and introduces dual read/write anchoring so decode/spec-decode can reuse cached SSM states without recomputing prefixes.

Changes:

Adds V2 runner plumbing to track and stage the previous step’s last-scheduled block anchor on-GPU, and plumbs it into Mamba2/GDN attention metadata builders for correct spec-decode reads after block crossings.
Extends GDN all-mode metadata + layer wiring: per-chunk intermediate-state export from the FLA chunk kernel, per-block checkpoint scatter during prefill, and dual-anchor state addressing in decode/spec-decode kernels.
Updates shared scheduling/config/platform validation to support kernel-chunk-aligned all-mode prefill splitting and fail-fast block-size constraints; adds extensive unit/kernel coverage.

Reviewed changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
vllm/v1/worker/gpu/model_states/mamba_hybrid.py	Adds V2 all-mode spec anchor tracking/staging and plumbs `prev_last_scheduled_idx` into metadata kwargs for Mamba2/GDN builders.
vllm/v1/worker/gpu_model_runner.py	Widens V1 runner plumbing to pass `prev_last_scheduled_idx` to the GDN metadata builder as well as Mamba2.
vllm/v1/core/sched/scheduler.py	Extends prefill splitting logic to support all-mode kernel-chunk clipping and disables align-only partial-hit logic in all-mode.
vllm/v1/core/kv_cache_utils.py	Refines EAGLE/MTP warning to reflect Mamba-group exemption semantics.
vllm/v1/core/kv_cache_coordinator.py	Adjusts EAGLE “flag all groups” fallback to exclude Mamba groups to preserve SSM prefix reuse.
vllm/v1/attention/backends/mamba_attn.py	Factors shared all-mode block-index computation for reuse by both Mamba and GDN metadata builders.
vllm/v1/attention/backends/gdn_attn.py	Adds all-mode request-level block index metadata, prev-step anchor handling, and persistent buffers for cudagraph-safe decode/spec decode.
vllm/third_party/flash_linear_attention/ops/fused_sigmoid_gating.py	Adds dual read/write state indexing and direct block-table addressing for all-mode decode/spec decode.
vllm/third_party/flash_linear_attention/ops/fused_recurrent.py	Adds direct block-table + anchor support to packed recurrent decode for all-mode dual-anchor semantics.
vllm/third_party/flash_linear_attention/ops/chunk.py	Adds optional intermediate-state export (`return_intermediate_states`) to support block-level checkpointing.
vllm/platforms/interface.py	Adds all-mode mamba block-size validation (must be kernel-chunk multiple) and configures scheduler prefill align size for GDN/FLA kernels.
vllm/model_executor/models/qwen3_next.py	Declares Qwen3-Next supports Mamba prefix caching and removes the previous all-mode rejection.
vllm/model_executor/models/qwen3_next_mtp.py	Treats all-mode as a no-op for the attention-only MTP head (target model still benefits under spec decode).
vllm/model_executor/layers/mamba/gdn/qwen_gdn_linear_attn.py	Forces Triton/FLA for all-mode prefill, implements all-mode prefill scatter and dual-anchor decode/spec-decode wiring.
vllm/model_executor/layers/mamba/gdn/all_mode_utils.py	New helper implementing per-block SSM checkpoint scatter using FLA intermediate states + final state.
vllm/config/vllm.py	Adjusts default prefix-cache retention behavior for mamba modes and adds block-size/budget validation for all-mode chunk-aligned split.
vllm/config/model.py	Resolves GDN mamba chunk size from `FLA_CHUNK_SIZE` (64) instead of Mamba1 default (2048).
vllm/config/cache.py	Adds `mamba_all_mode_prefill_align_size` to CacheConfig and includes it in cache hashing.
tests/v1/worker/test_mamba_utils.py	Adds V1 contract tests for all-mode prev-step anchor tracking buffer semantics.
tests/v1/worker/test_mamba_hybrid_prev_anchor.py	New CUDA test validating the V2 Triton kernel that stages + updates per-request write anchors.
tests/v1/core/test_prefix_caching.py	Updates mocks to include `mamba_cache_mode` where required by the scheduler split logic.
tests/v1/core/test_mamba_chunk_size.py	New tests for GDN chunk-size resolution and all-mode block-size validation.
tests/v1/core/test_mamba_block_size_validation.py	New validation tests for scheduler-budget constraints in all-mode chunk-aligned split.
tests/v1/core/test_mamba_all_mode_split.py	New scheduler split tests ensuring all-mode chunk-end clipping behavior and alignment invariants.
tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py	Updates mocks to include `mamba_cache_mode` where required by the scheduler split logic.
tests/v1/attention/test_gdn_metadata_builder.py	Adds all-mode metadata tests (block indices, prev-step anchor where-semantics, cudagraph buffers).
tests/models/test_qwen3_next_prefix_caching.py	New tests asserting Qwen3-Next declares prefix-caching support and config defaults/behavior are correct.
tests/kernels/mamba/test_gdn_scatter.py	New CPU test validating block checkpoint scatter mapping and alignment skipping behavior.
tests/kernels/mamba/test_gdn_all_mode_spec_decode.py	New CUDA tests validating all-mode dual-anchor speculative decode matches align where applicable and handles crossings.
tests/kernels/mamba/test_gdn_all_mode_prefill.py	New CUDA tests validating all-mode prefill parity vs align + checkpoint writing behavior and backend resolution.
tests/kernels/mamba/test_gdn_all_mode_decode.py	New CUDA tests validating all-mode dual-anchor decode across packed/unpacked and mixed batches.
tests/kernels/mamba/test_fused_sigmoid_gating_dual_index.py	New CUDA tests validating dual-index read/write isolation and non-contiguous index normalization.
tests/kernels/mamba/test_fla_chunk_block_states.py	New CUDA parity tests pinning the semantics of FLA intermediate state export (`h`) used for checkpointing.

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+            if end < last_aligned:
+                # Re-align a misaligned start (e.g. externally loaded KV)
+                # at the next boundary; otherwise clip the end down.
+                if start % align != 0:
+                    end = min(end, (start // align + 1) * align)
+                else:
+                    end = end // align * align


+    if (ssm_state_indices_output is not None
+            and ssm_state_indices_output.stride(-1) != 1):
+        ssm_state_indices_output = ssm_state_indices_output.contiguous()


+    for s in range(num_prefills):
+        first = int(block_idx_first_scheduled_token_p[s])
+        last = int(block_idx_last_scheduled_token_p[s])
+        n = last - first
+        if n <= 0:
+            continue
+        ncomp = int(num_computed_tokens_p[s])
+        if ncomp % chunk_size != 0:
+            # Not chunk-aligned: cannot map interior block boundaries to exact FLA
+            # chunk states. Skip (never write an approximate checkpoint into APC).
+            continue
+        cache_blocks = block_table_p[s, first:last]
+        fc = int(first_chunk_p[s])
+        # This sequence's chunks occupy h[fc : seq_hi_excl); clamp the gather to that
+        # per-sequence range (not just the global [0, nt)) so a would-be overshoot can
+        # never bleed into an adjacent sequence's chunks. Exact indices already fall in
+        # range when block boundaries are chunk-aligned; this is defense-in-depth.
+        seq_hi = (
+            int(first_chunk_p[s + 1]) - 1
+            if (s + 1) < first_chunk_p.shape[0]
+            else nt - 1
+        )


In all-mode, a misaligned prefill chunk start can still happen (e.g. externally loaded KV). The current logic only realigns when end < last_aligned, so if the final prefill bite reaches last_aligned it may proceed with a misaligned start and skip materializing exact per-block checkpoints inside that bite. Consider always realigning to the next align boundary when start % align != 0 and that boundary is <= last_aligned, even for the final bite; then allow the last bite (from an aligned start) to end anywhere. Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com> Signed-off-by: Anurag Dutt <anurag2709@gmail.com>

mergify · 2026-09-06T06:09:18Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @anuragdutt.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

anuragdutt added 7 commits August 31, 2026 12:30

Tests for GDN all-mode prefix caching

d758c5b

Signed-off-by: Anurag Dutt <andutt@nvidia.com>

Lint: SPDX headers, import guards, self-contained test docstrings, ru…

3bba69d

…ff format Signed-off-by: Anurag Dutt <andutt@nvidia.com>

mergify Bot added qwen Related to Qwen models mrv2 Model Runner V2 specific scheduler kv-cache-manager labels Aug 31, 2026

Merge branch 'main' into gdn-allmode-v2

05a233e

anuragdutt marked this pull request as ready for review September 1, 2026 02:58

Copilot AI lite review requested due to automatic review settings September 1, 2026 02:58

anuragdutt requested review from ApostaC, DarkLight1337, WoosukKwon, ZJY0516, alexm-redhat, heheda12345, ivanium, njhill, orozery, robertgshaw2-redhat, sighingnow, vadiklyutiy and ywang96 as code owners September 1, 2026 02:58

anuragdutt requested review from AndreasKaratzas, mgoin, tlrmchlsmth, yewentao256 and zyongye as code owners September 1, 2026 02:58

claude Bot reviewed Sep 1, 2026

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anuragdutt requested review from LucasWilkinson, MatthewBonanni, ProExpertProg, houseroad, tdoublep, tomeras91 and youkaichao as code owners September 1, 2026 02:58

Copilot started reviewing on behalf of anuragdutt September 1, 2026 02:59 View session

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anuragdutt and others added 2 commits September 1, 2026 02:18

Merge branch 'main' into gdn-allmode-v2

4e0d36d

This was referenced Sep 2, 2026

[Bugfix] OffloadingConnector: stop zeroing offload hits under MTP/EAGLE spec decode #52771

Merged

[Bugfix][V1] Register accepted spec-decode tokens before terminal cleanup #54979

Open

zyongye mentioned this pull request Sep 2, 2026

[Perf][Mamba] Skip post-step state bookkeeping without speculative decoding #55025

Closed

anuragdutt mentioned this pull request Sep 4, 2026

[Perf][Model Runner V2] GDN all-mode decode-path optimizations: packed QKV, fused index prep, weight de-interleave + GEMM fold #55260

Open

mergify Bot added the needs-rebase label Sep 6, 2026

nicholaskh-ai mentioned this pull request Sep 8, 2026

[Feature][Config] Support application-directed Mamba checkpoint token and input processing #55873

Open

Pinoeer-kingxi mentioned this pull request Sep 14, 2026

[Bugfix][Core] Cache final multimodal boundaries for Mamba siblings #56818

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[Model Runner V2] Automatic prefix caching for GDN hybrid models (mamba_cache_mode=all) with MTP speculative decoding - #54637

[Model Runner V2] Automatic prefix caching for GDN hybrid models (mamba_cache_mode=all) with MTP speculative decoding#54637
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anuragdutt commented Aug 31, 2026

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