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Tests only, no production change. Both run CPU-only through the existing
make_kv_cache_manager / make_request harness -- no model execution, no GPU.
11 passed, 7 xfailed
The seven are real failures today, carried as xfail(strict=True) so CI stays green.
When one is fixed the test XPASSes and strict turns that into a failure, so the
marker has to be removed with the fix.
1. The EAGLE drop costs a whole block, not one hash unit
tests/v1/core/prefix_cache/test_eagle_mamba_drop_cost.py
prefix_match_unit exists so the drop costs one hash unit rather than one cache
block. It does that for full attention, but not for the reconciled hit. Block 16,
hash unit 4, producer cached 48 tokens, consumer sharing the prefix:
full attention alone 48 -> 44 one hash unit, as intended
mamba alone 48 MambaManager ignores drop_eagle_block
reconciled 32 a whole block
Neither group gives up a block on its own. It is given up when the two are
combined: mamba is capped at the attention candidate (44), its state exists only
at 16/32/48, so its lookup falls back to 32. The xfail would pass if a mamba
state were cacheable one hash unit below a block boundary, which is where EAGLE
resumes.
2. Sparse mamba keys against a dense-grid event walk
tests/v1/core/prefix_cache/test_mamba_align_sparse_keys.py
Interior mamba blocks are null and carry no block hash; reachable_block_mask
skips them deliberately. But emit_cached_block_events walks the hit list by
position and publishes one key per hit block, so for the mamba group it publishes
keys that were never stored:
stored by the mamba group : hash @ 80 tokens
published in the event : hash @ 64 (interior, null, never stored)
hash @ 128 (past the end of an 80-token hit)
On a prompt that is not mamba-block-aligned the same walk runs off the end:
kv_cache_manager.py:279 get_computed_blocks
block_pool.py:406 emit_cached_block_events
kv_cache_utils.py:2315 BlockHashListWithBlockSize.__getitem__
IndexError: list index out of range
Needs kv_cache_report_mode="full", and reproduces with no prefix_match_unit set
at all, so it is not specific to partial matching. The mooncake connector solves
the same problem correctly -- it emits a positional boolean mask and marks null
blocks False rather than assuming a key exists at every slot.
Every xfail is checked in both directions: xfailed as written, and XPASS(strict)
under a stand-in fix. A strict marker that cannot flip asserts nothing. The crash
test also carries raises=IndexError so an unrelated exception cannot keep it
green.
3. The same drop, when the shared prefix ends before the owner's prompt
tests/v1/core/prefix_cache/test_eagle_mamba_shared_system_prompt.py
Defect 1 uses a producer whose whole prompt is the shared prefix. A served system
prompt is the other shape: every request opens with the same preamble and then
diverges, so the producer's prompt tail is over tokens no consumer shares. Block
16, hash 4, producer prompt 40 = 24 shared + 16 private, consumer sharing 24:
full attention alone 16 -> 12 one hash unit below the shared boundary
mamba alone 16 MambaManager ignores drop_eagle_block
reconciled 0 everything
The shared prefix ends mid-block, so full attention stops at the last whole
shared block (16) and the drop takes it to 12. Mamba has state at 0 and 16 only;
capped at 12 it falls back to 0 and the consumer recomputes the entire prefix.
Without EAGLE the same consumer keeps 16.
This is the shape a check-point at the producer's prompt *tail* cannot serve: 40,
and 36 one unit below it, are both over the private suffix. The state has to
exist one hash unit below the shared block boundary.
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…ibling resumes Second of the two EAGLE + `--mamba-cache-mode align` prefix-cache defects pinned by vllm-project#52371. Full attention hits where it holds a key; EAGLE prunes one hash unit off that candidate and drops it. Mamba materializes state only on its own block grid, so nothing exists at the pruned position and the hit floors back a whole Mamba block -- on a 4,416-token block a 64-token drop costs 4,416 tokens of reuse, per request. vllm-project#51295 (commit 1 here) covers the case where the shared prefix runs to the end of the producer's prompt. When it ends earlier -- a system prompt followed by a per-request suffix, the deployed shape -- the producer's tail sits over tokens no sibling shares, so a check-point there is unreachable. The sibling stops at the last shared boundary and EAGLE drops one hash unit below it; that position is `request.shared_prefix_boundary`, already recorded by `get_computed_blocks`. So: the scheduler ends a chunk at the junction as observed rather than floored to the block grid, which rounds the resume point away, and `MambaManager` accepts the junction in addition to the prompt tail. The two must agree -- the junction stop is the earliest mandatory stop, so it REPLACES the block-boundary stop, and a junction the manager then refuses leaves less cached than not splitting at all. `KVCacheManager` therefore computes one predicate that both sides read, rather than each deriving its own: - EAGLE must be annotated on the model. The manager takes the drop per KV cache group while the scheduler flag is model-wide, so deriving them separately lets the split fire where the manager will not check-point. - Fine-grained partial hash hits must be on, or nothing can look the entry up. - No multi-module MTP. `cache_blocks` then hands the manager `num_computed - num_reprefillable`, not the chunk end, so the position it would publish is not the one it holds. Registration is bounded to the prompt chunk being computed: during decode the target is the running state block, mutated in place, which equals what its key promises only after that step's forward. A junction past the prompt falls back to the stock block-floored stop rather than being dropped. The manager writes nothing past the prompt, but stock vLLM still check-points at the block boundary, so zeroing the stop lost a snapshot a resumed request's siblings could have reused. Off by default behind `--enable-mamba-fine-grained-prefix-cache` (`CacheConfig.enable_mamba_fine_grained_prefix_cache`), listed in `compute_hash`'s ignored factors alongside the other prefix-cache knobs so it does not perturb the compiled-graph cache. Signed-off-by: Adam Shaver <ashaver@nvidia.com> Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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…ibling resumes Second of the two EAGLE + `--mamba-cache-mode align` prefix-cache defects pinned by vllm-project#52371. Full attention hits where it holds a key; EAGLE prunes one hash unit off that candidate and drops it. Mamba materializes state only on its own block grid, so nothing exists at the pruned position and the hit floors back a whole Mamba block -- on a 4,416-token block a 64-token drop costs 4,416 tokens of reuse, per request. vllm-project#51295 (commit 1 here) covers the case where the shared prefix runs to the end of the producer's prompt. When it ends earlier -- a system prompt followed by a per-request suffix, the deployed shape -- the producer's tail sits over tokens no sibling shares, so a check-point there is unreachable. The sibling stops at the last shared boundary and EAGLE drops one hash unit below it; that position is `request.shared_prefix_boundary`, already recorded by `get_computed_blocks`. So: the scheduler ends a chunk at the junction as observed rather than floored to the block grid, which rounds the resume point away, and `MambaManager` accepts the junction in addition to the prompt tail. The two must agree -- the junction stop is the earliest mandatory stop, so it REPLACES the block-boundary stop, and a junction the manager then refuses leaves less cached than not splitting at all. `KVCacheManager` therefore computes one predicate that both sides read, rather than each deriving its own: - EAGLE must be annotated on the model. The manager takes the drop per KV cache group while the scheduler flag is model-wide, so deriving them separately lets the split fire where the manager will not check-point. - Fine-grained partial hash hits must be on, or nothing can look the entry up. - No multi-module MTP. `cache_blocks` then hands the manager `num_computed - num_reprefillable`, not the chunk end, so the position it would publish is not the one it holds. Registration is bounded to the prompt chunk being computed: during decode the target is the running state block, mutated in place, which equals what its key promises only after that step's forward. A junction past the prompt falls back to the stock block-floored stop rather than being dropped. The manager writes nothing past the prompt, but stock vLLM still check-points at the block boundary, so zeroing the stop lost a snapshot a resumed request's siblings could have reused. Off by default behind `--enable-mamba-fine-grained-prefix-cache` (`CacheConfig.enable_mamba_fine_grained_prefix_cache`), listed in `compute_hash`'s ignored factors alongside the other prefix-cache knobs so it does not perturb the compiled-graph cache. Signed-off-by: Adam Shaver <ashaver@nvidia.com> Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This was referenced Sep 10, 2026
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Tests only, no production change. Three properties of mamba
align+ EAGLE/MTP prefix caching, pinned as executable assertions so a fix can be judged against them.CPU only: drives the KV-cache manager through the existing
make_kv_cache_manager/make_requestharness. No model execution, no GPU.How to read the table. A test that passes on
mainis pinning behaviour as it is today — it goes red if that behaviour changes, intentionally or not. A test marked xfail is asserting the behaviour we want and does not get it; it carriesxfail(strict=True), so CI stays green now, and when a fix lands the test XPASSes andstrictturns that into a failure — the marker has to be removed together with the fix. Nothing here is expected to be red onmain.1. The EAGLE drop costs a whole block, not one hash unit
tests/v1/core/prefix_cache/test_eagle_mamba_drop_cost.py— block 16, hash unit 4, producer caches 48 tokens (three whole blocks), consumer shares all 48.prefix_match_unitexists so the drop costs one hash unit rather than one cache block. It does that for full attention, but not for the reconciled hit: mamba is capped at the attention candidate (44) and its state exists only at 16/32/48, so its lookup falls back to 32.maintest_full_attention_gives_up_only_one_hash_unit48 - 4 = 44— the drop is one hash unit, as designedtest_mamba_alone_reaches_the_full_prefix48—MambaManagerignoresdrop_eagle_blocktest_without_eagle_nothing_is_given_up48, isolating the drop as the causetest_reconciled_hit_should_not_give_up_a_whole_block44; it is32. Also asserts every group returned enough blocks to cover the hitNeither group gives up a block on its own — it is given up in the reconciliation.
2. Sparse mamba keys walked as if they were a dense grid
tests/v1/core/prefix_cache/test_mamba_align_sparse_keys.py— parametrised over two shapes,[toy](hash 2, attention block 2, mamba block 4) and[large-block](hash 16, attention block 64, mamba block 64), so nothing depends on one set of sizes.Under
align, interior mamba blocks are null and carry no key on purpose — the group's only hittable positions are chunk ends.emit_cached_block_eventswalks the hit list positionally, as though every index carried a key.maintest_mamba_group_grid_is_sparse[toy]/[large-block]test_emit_cached_block_events_current_shape[toy]/[large-block]token_idsspan twice the length of the hit. Unmarked, so it goes red the moment the shape changestest_emit_cached_block_events_invents_mamba_keys[toy]/[large-block]test_emit_cached_block_events_indexerror_on_unaligned_prompt[toy]/[large-block]IndexErrorinsideget_computed_blocks. Pinned withraises=IndexErrorso a different exception is a failure, not a silent passtest_plain_mamba_align_no_partial_hash_also_invents_keysprefix_match_unitset at all (hash_block_size == block_size), so this is not a partial-matching-only problemRequires
kv_cache_report_mode="full"to observe, except the last, which needs onlyalign.3. The same drop, when the shared prefix ends before the producer's prompt
tests/v1/core/prefix_cache/test_eagle_mamba_shared_system_prompt.py— block 16, hash unit 4, producer prompt 40 = 24 shared + 16 private, consumer shares the first 24.Section 1 uses a producer whose whole prompt is the shared prefix. A served system prompt is the other shape: every request opens with the same preamble and then diverges, so the producer's prompt tail sits over tokens no consumer shares. The shared prefix ends mid-block (
24 % 16 = 8), so full attention stops at the last whole shared block (16) and the drop takes it to 12; mamba has state at 0 and 16 only, and capped at 12 it can only fall back to 0.maintest_full_attention_gives_up_only_one_hash_unit16 - 4 = 12test_mamba_alone_reaches_the_shared_block_boundary16test_without_eagle_the_hit_survives16test_reconciled_hit_is_currently_nothing0— the consumer recomputes the entire shared prefix. Unmarked, so it goes red when this improvestest_reconciled_hit_should_reach_the_resume_point12, the position EAGLE resumes fromHere the drop does not cost a block — it costs the entire hit, while both groups independently reach at least 12.
The distinction between this and section 1 matters for where a fix puts the state: a check-point at the producer's prompt tail cannot serve this shape, because 40 — and 36, one hash unit below it — are both over the private suffix. The state has to exist one hash unit below the shared block boundary.