[Bugfix][KV Cache] Fix hybrid block size alignment across heterogeneous TP

[dilberx]

Purpose

Related to #41037: #49612 closed that issue by making the NIXL connector
tolerate heterogeneous block sizes (block_size_ratio != 1) between prefill
and decode. This PR fixes the root cause. _align_hybrid_block_size
computes the Mamba-to-attention page-size ratio from per-rank page sizes,
so for hybrid attention/Mamba models the aligned block size depends on TP
size. That's still true after #49612; heterogeneous P/D just no longer
crashes on it.

This computes the ratio from TP-stable page sizes while keeping the final
padding check against the local per-rank page size, so:

• prefill/decode TP configs agree on block size in the common case
• prefix-cache granularity stops shifting with TP for a fixed model/config

I checked for duplicate open PRs for #41037 and did not find one addressing
the root cause (#49612 works around it in the connector layer). I used AI
assistance while developing and validating this change, and reviewed the
resulting diff before submitting.

Test Plan

• python -m pytest tests/platforms/test_hybrid_block_size.py -q
• python -m pytest --confcutdir=tests/v1/kv_connector/unit tests/v1/kv_connector/unit/test_nixl_connector_hma.py -q
• python -m pytest "tests/models/language/generation/test_hybrid.py::test_apc_single_prompt[1-5-2-64-ai21labs/Jamba-tiny-dev]" -q -s

Test Result

Re-ran all three against current main (post rebase) to confirm nothing drifted:

tests/platforms/test_hybrid_block_size.py

1 passed, 3 warnings in 0.25s

tests/v1/kv_connector/unit/test_nixl_connector_hma.py

92 passed, 8 warnings in 149.56s

tests/models/language/generation/test_hybrid.py::test_apc_single_prompt[1-5-2-64-ai21labs/Jamba-tiny-dev]

1 passed, 5 warnings in 109.28s

All warnings across all three runs are pre-existing/environmental
(PyTorch/SWIG deprecations, a zmq-context teardown ResourceWarning, the
comparison test's own diagnostic UserWarning). None originate in this
diff. Confirmed the aligned block size is TP-stable in the running engine
too: kv cache group sizes [144, 144, 144, 144, 144, 144, 144, 144], uniform
across all groups.

The exact heterogeneous P/D repro with nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-FP8 (prefill TP=8 / decode TP=1) was not run locally because it requires a larger multi-GPU setup than the available RTX 3080. Already asked in the team Slack for a run on multi-GPU hardware to confirm this case directly.

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[dilberx]

Rebased onto current main. One fix was needed: main added AttentionBackend.customize_spec, which _align_hybrid_block_size now calls, so the test's fake backend needed the stub. It returns the spec unchanged, like the base class default.

Test results on current main:

pytest tests/platforms/test_hybrid_block_size.py -v
1 passed

ruff check / ruff format --check
All checks passed! / 2 files already formatted

tests/platforms/test_cpu.py has 8 failures, but they reproduce on clean main with this branch unapplied, so they are unrelated.

Also booted a real hybrid model to confirm the aligned block size still yields a working KV cache (tiiuae/Falcon-H1-0.5B-Base, single GPU, eager):

resolved block_size: 800
prompt: 'The capital of France is'
output: ' Paris.\nThe capital of France is Paris.'
prompt: 'def fibonacci(n):'
output: '\n    if n <= 1:\n        return n\n    else:\n        return fibonacci(n'

The 800 confirms the alignment path ran. That run is TP=1, so it does not cover the heterogeneous-TP case this PR targets; the unit test covers that by varying TP.

[khluu]

/ci run

[github-actions]

✅ Triggered Buildkite CI #89596 for commit 2a8d5c9d2129.

[dilberx]

cc @tdoublep @tomeras91 for review when you get a chance. This addresses the TODO in _align_hybrid_block_size for hybrid cache block alignment across heterogeneous TP.

[dilberx]

@njhill following up from the Slack thread.

Beyond the crash #49612 fixed, a TP-stable block_size removes work from the decode critical path. block_size_ratio > 1 gates kv_postprocess_blksize_on_receive in base_worker.py, which runs index_select, permute and reshape over the KV cache per request, per layer. Make block_size TP-stable and the common heterogeneous-TP P/D case lands on block_size_ratio == 1 and skips it.

The cause is in _align_hybrid_block_size: it derives the Mamba-to-attention page-size ratio from per-rank page sizes, which is not TP-invariant, since Mamba shard padding means state does not scale as local * TP. The same model and config gets a different block size at TP=2 than at TP=4. #49612 made the connector tolerate that; the ratio is unchanged. Prefix-cache granularity moves with it too, which hits anyone running hybrid models with APC, disaggregation or not.

This computes the ratio from TP-stable page sizes. TP 1/2/4/8/16 agree in the unit test, CI build 89596 is green, and three test runs against current main are in the description.

I cannot cover prefill TP=8 and decode TP=1 on Nemotron with a single RTX 3080, and have asked for a multi-GPU run in #feat-prefill-disaggregation.

Would you take a look?

[dilberx]

@khluu thanks for the earlier CI run. Is there a way to get the distributed PD jobs onto this PR?

The case I cannot cover locally is a hybrid model with prefill TP=8 and decode TP=1, checking both sides agree on block_size at startup.