[Bugfix] Stop mamba align prefill at the replay boundary - #50409
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_mamba_block_aligned_split derives its mandatory chunk stops from last_cache_position = num_tokens - num_tokens % block_size. When the prompt length is an exact multiple of the block size that is num_tokens itself, so the prefill runs as one chunk and the only Mamba state materialized sits at num_tokens. get_computed_blocks caps every lookup at num_tokens - 1, one token below it, so the Mamba group reports a 0-token hit and the reconciled hybrid hit collapses to 0 even though full attention has the whole prefix cached. Add the replay boundary, (num_tokens - 1) // block_size * block_size, as another mandatory stop, clamped to last_cache_position. It only differs from an existing stop when num_tokens is an exact multiple of block_size, and is always a duplicate under eagle, which already backs last_cache_position off by a block. With a full attention + Mamba align KV cache config (block 16), a 32-token prompt replayed as a second request went from a 0-token local prefix hit to 16, and 48 to 48 went from 0 to 32. Producer lengths that are not exact multiples are unchanged, and mamba_cache_mode="all" is unaffected.
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This pull request has merge conflicts that must be resolved before it can be |
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Rebased onto current main (38a267c). The only conflict was in #50000 also reworked Re-verified on the new base, CPU only:
Unchanged from the original submission: no GPU, multi-TP, fp8 KV cache or real |
…ppend conflict resolved by keeping both)
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Fixes #50235
_mamba_block_aligned_splitpicks its mandatory chunk stops fromlast_cache_position = num_tokens - num_tokens % block_size. When the prompt length is an exact multiple of the blocksize that is
num_tokensitself, so the prefill runs as a single chunk and, inalignmode whereSSM state only materializes at chunk ends, the only Mamba state cached sits at
num_tokens.get_computed_blockscaps every lookup atnum_tokens - 1, one token below it, so the Mamba groupreports a 0-token hit and the reconciled hybrid hit is 0 even though full attention has the whole
prefix.
The patch adds the replay boundary,
(num_tokens - 1) // block_size * block_size, as anothermandatory stop. It is clamped to
last_cache_position, so it only differs from an existing stopwhen
num_tokens % block_size == 0, and it is always a duplicate under eagle (which already backslast_cache_positionoff by a block). Cost is one extra scheduling step for prompts whose length isan exact multiple of the block size.
Verified on CPU only, driving a real
Schedulerwith a full attention + MambaalignKV cacheconfig (block 16, prefix caching on, no connector): a producer prompt of 32 tokens followed by the
same 32-token prompt went from a 0-token local prefix hit to 16, and 48 -> 48 went from 0 to 32.
Producer lengths 31 and 33 were already fine and are unchanged, and
mamba_cache_mode="all"isunaffected. A sweep over producer lengths 1-66, consumer lengths P, P+1, P+2, block/hash sizes
16/16 and 16/4, both cache modes, two token budgets and two pool sizes shows no other change.
Tests run:
tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py-> 18 passed. The new testtest_mamba_align_split_stops_at_replay_boundaryfails with the scheduler change reverted(
assert 4096 == 3584).tests/v1/core/test_prefix_caching.py,tests/v1/core/test_single_type_kv_cache_manager.py,tests/v1/core/test_kv_cache_utils.py,tests/v1/core/prefix_cache/-> 198 passed, 1 failurethat only reproduces with
VLLM_LOGGING_LEVEL=ERRORset (a caplog assertion) and passes without it.tests/v1/core/test_scheduler.py-> 136 passed, 1 failure(
test_async_scheduling_pp_allows_rescheduling_with_output_placeholders) that reproduces on aclean tree.
ruff checkandruff format --checkclean on both changed files.Not verified: nothing here ran on a real Kimi-K3 deployment, on 8x TP, with fp8 KV cache, or with
the
OffloadingConnector, so no model eval is included. The change only moves a chunk boundary anddoes not touch what is computed, so model output is unchanged by construction.
One caveat on the reported reproduction. Rows 1 and 4 of the table in the issue reproduce exactly.
Rows 2 and 3 (producer 3072, consumer 3073 / 3074) do not: a consumer of
P+1tokens has a lookupcap of exactly
P, so it still reaches the state atPboth before and after this patch. The caseI do reproduce has the same trigger, a producer whose prompt length is an exact multiple of the
block size, with the consumer's cap landing below that boundary. There may be a second cause behind
rows 2 and 3 that this patch does not address.
Thanks to the reporter for the length table, which is what made the boundary condition obvious.
AI assistance was used to investigate and write this change.
Duplicate check: no open PR references #50235. Two open PRs do touch
_mamba_block_aligned_splitand neither covers this. #45477 fixes the eagle prune zeroinglast_cache_positionon short prompts, which lets chunk ends drift off the block grid and poisonsthe cache under spec decode; that is a correctness bug on a different branch of the same function.
#48815 adds an opt-in MTP retention tweak behind
VLLM_MAMBA_ALIGN_RETAIN_MTP_CACHE_BLOCKandexplicitly leaves prompts ending exactly at a block boundary on the existing eagle backoff. This
change is on the non-eagle path and adds a stop that neither PR adds.