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NVFP4 marlin disagrees with itself on sm_110: test_ops_moe_grouped block8-vs-block16 bitdiff=15/32768 on an ENABLED kernel #962

Description

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What

On Jetson Thor (sm_110), tests/vt/test_ops_moe_grouped.cpp:1144 fails:

NVFP4 block8-vs-block16  M=8 K=4096 N=4096  bitdiff=15/32768

Two NVFP4 groupings that should agree disagree in 15 of 32768 output elements.

Why this one is different from the rest of the sm_110 baseline

#955 records 14 failures at 5a0ffe9e3, and thirteen of them are feature
absence
: no FP8 op kernel, no vendored FA-2 on MLA prefill, two tests
hardcoding GB10. Those cannot be made green on this architecture and are not
defects in anything.

This one is not absence. marlin-nvfp4 is ENABLED for [110] — verified
from the arch gating, alongside fa2, cutlass-nvfp4{,-sm100}, cutlass-fp8,
scaledmm-c3x-* and fp4-mma, which are all disabled for that arch. So the
kernel under test is present and running, and it is disagreeing with itself
across two block sizes. That is a correctness signal on a live path.

Filed separately because #955 is a baseline issue, and a baseline issue is the
kind of record that gets closed wholesale as "known reds on this host". This
finding should not go with it. #955 states the reasoning in its text but not as
its own defect; it should point here instead.

Not established

  • Whether GB10 (sm_121) shows the same disagreement. If it does, this is not
    sm_110-specific and the title is wrong.
  • Which of the two groupings is correct, or whether both are wrong.
  • Whether 15/32768 is a rounding-boundary artifact or a genuine indexing or
    accumulation defect. Bit-count alone does not distinguish those, and a small
    bitdiff is not evidence of a small problem.

Reproduce with the container recipe in .agents/environment.md (Thor section,
landing in #956); the test is test_ops_moe_grouped.

Found during #699 W0.5, the Thor bring-up.

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