[None][fix] Size the FlashInfer page-index buffer for VSWA on V1 KV cache - #18975
[None][fix] Size the FlashInfer page-index buffer for VSWA on V1 KV cache#18975brnguyen2 wants to merge 1 commit into
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FlashInferAttentionMetadata read kv_cache_manager.blocks_in_primary_pool
directly. KVCacheManagerV2 always exposes that scalar, but the V1
KVCacheManager only assigns it when it ends up with a single pool: the VSWA
sizing path fills blocks_per_window instead and leaves the scalar unset, as
the FIXME beside it in pyexecutor/resource_manager.py already notes ("only
covers the single window case and not VSWA scheme").
Any VSWA model routed to a V1 manager therefore died in metadata
construction with
AttributeError: 'KVCacheManager' object has no attribute
'blocks_in_primary_pool'
Disaggregated serving is one such route: a model preference for V2 is
demoted to V1 outside the NIXL + Python-transceiver combination, so a VSWA
model that runs fine aggregated crashed as soon as a cache transceiver was
configured.
Route the read through a _get_blocks_in_primary_pool() helper that uses the
scalar when present and otherwise falls back to the largest per-window
primary count from blocks_per_window. That is an upper bound across the
pools, so the buffer stays large enough for each of them, and it feeds the
same max_num_blocks seed the per-layer buffer sizes are folded into. Raise a
named error instead of an anonymous AttributeError when neither attribute
is available.
Adds a CPU-only unit test covering the V2 scalar, the V1 VSWA table, and
the neither-attribute error.
Signed-off-by: Brian Nguyen <brnguyen@nvidia.com>
WalkthroughFlashInfer now resolves primary KV-pool block counts from either manager-wide sizing or per-window sizing. Buffer initialization uses this compatibility path. Tests cover both manager versions and missing sizing attributes. ChangesFlashInfer primary pool sizing
Priority: ➖ Normal Estimated code review effort: 2 (Simple) | ~10 minutes Severity of issue fixed: Medium Merge Risk: 🔵 Low · up to This change enables V1 VSWA cache managers to size FlashInfer page-index buffers from per-window block counts. The fallback logic is covered, but metadata construction using that fallback lacks an end-to-end regression test, leaving a bounded risk of initialization failures or incorrectly sized buffers. Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@tensorrt_llm/_torch/attention/backends/flashinfer.py`:
- Around line 1099-1100: Add a regression test for
FlashInferAttentionMetadata._post_init_with_buffers() using a V1 VSWA-shaped
manager that provides only blocks_per_window, then assert
_paged_kv_indices.numel() matches the largest primary-pool count. Ensure the
test exercises the metadata-construction call site rather than calling
_get_blocks_in_primary_pool directly.
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tensorrt_llm/_torch/attention/backends/flashinfer.pytests/unittest/_torch/attention/test_flashinfer_attention.py
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Description
FlashInferAttentionMetadata._post_init_with_buffersreadkv_cache_manager.blocks_in_primary_pooldirectly to seedmax_num_blocks,which sizes the flat
_paged_kv_indicesbuffer and the per-pool VSWA buffers.That scalar is not part of the KV cache manager interface:
KVCacheManagerV2always exposes it as a property.KVCacheManageronly assigns it when it ends up with a single pool.The VSWA sizing path (
calculate_max_num_blocks_for_vswa()) fills theblocks_per_windowdict instead and leaves the scalar unset, as the standingFIXMEnext to it inpyexecutor/resource_manager.pyalready notes ("onlycovers the single window case and not VSWA scheme").
Any VSWA model routed to a V1 manager therefore dies in metadata construction
with:
Disaggregated serving is one such route:
llm_utils.pykeeps a model's V2preference only for the NIXL + Python-transceiver combination and otherwise
falls back to V1, so a VSWA model that runs fine aggregated crashes as soon as a
cache transceiver is configured.
The fix routes the read through a small
_get_blocks_in_primary_pool()helper:blocks_in_primary_poolwhen the manager exposes it (V2, single-pool V1);blocks_per_window. That is anupper bound across the pools, so the buffer stays large enough for every
pool, and it feeds the same
max_num_blocksseed that the per-layer buffersizes are folded into;
AttributeErrordescribing both missing attributes whenneither is available, instead of an anonymous attribute read.
This matches the
getattr-guarded V2-only lookups a few lines further down inthe same function ("V1 managers lack the per-pool infrastructure"). The
FIXMEinresource_manager.pyis left in place; its other point(single-window-only semantics of the scalar) still stands.
Test Coverage
Unit test (CPU-only, stub managers):
tests/unittest/_torch/attention/test_flashinfer_attention.py::TestFlashInferAttention::test_blocks_in_primary_pool_falls_back_to_blocks_per_windowblocks_per_windowtableis also present;
per-window primary count;
blocks_per_window, raises anAttributeErrorthat names the manager type and the missing attributes.The failure this fixes was first hit in disaggregated serving of a VSWA model
whose class prefers the V2 KV cache manager but was demoted to V1: engine setup
aborted during metadata construction with the
AttributeErrorabove. Thischange removes that crash by sizing the buffer from the per-window table when
the scalar is absent; the guard itself is covered by the unit test above.
PR Checklist
Please review the following before submitting your PR:
PR description clearly explains what and why. If using CodeRabbit's summary, please make sure it makes sense.
PR Follows TRT-LLM CODING GUIDELINES to the best of your knowledge.
Test cases are provided for new code paths (see test instructions)
If PR introduces API changes, an appropriate PR label is added - either
api-compatibleorapi-breaking. Forapi-breaking, includeBREAKINGin the PR title.Any new dependencies have been scanned for license and vulnerabilities
CODEOWNERS updated if ownership changes
Documentation updated as needed
Update tava architecture diagram if there is a significant design change in PR.
The reviewers assigned automatically/manually are appropriate for the PR.
Please check this after reviewing the above items as appropriate for this PR.
Dev Engineer Review
FlashInferAttentionMetadatanow supports both V2 scalar sizing and V1 VSWA per-window sizing. The fallback selects the largest primary-pool count and raises a descriptiveAttributeErrorwhen sizing data is missing. The change is localized and does not alter public APIs.QA Engineer Review
The CPU-only unit test adds coverage for V2 scalar lookup, V1 VSWA fallback, and missing-attribute errors. Coverage is sufficient for the changed logic. No integration test-list changes apply.
Per-File QA Perspective
tensorrt_llm/_torch/attention/backends/flashinfer.py: Verify metadata construction for V2 managers, V1 VSWA managers, and managers without either sizing attribute. Confirm disaggregated serving no longer fails withAttributeError.tests/unittest/_torch/attention/test_flashinfer_attention.py: Covers scalar lookup, maximum per-window fallback, and error handling. This unit test is not expected in integrationtest-db/or manual-QA lists.