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[https://nvbugs/6919819][perf] Autotune the CuTeDSL FC2 N tile again when the override is unset - #20277
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WalkthroughThe FC2 N-tile override now defaults to ChangesFC2 N-Tile Autotuning
Priority: ⬆️ High Estimated code review effort: 1 (Trivial) | ~5 minutes Change: Bug fix Suggested reviewers: Merge Risk: 🔵 Low · up to
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Check name Status Explanation Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Title check The title clearly identifies the NVBug, performance context, and primary change: restoring CuTeDSL FC2 autotuning when the override is unset. Description check The description explains the regression, root cause, solution, performance impact, and test coverage. It includes the required Description and Test Coverage sections. The checklist is only partially c…
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Review comments at @tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py:
- Around line 3215-3221: Update _allows_large_m_n256() to exclude runners where
use_expert_counts is enabled, so expert-capacity output rows cannot trigger N256
based on the token threshold; preserve the existing eligibility checks for other
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Review comments at @tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py:
- Around line 3188-3197: Update the FC2 runner identity or equivalent autotuner
cache-key discriminator to include the current N256 candidate eligibility
represented by the predicate using `fc2_n_tile_size_is_explicit`, `tile_size`,
and `use_expert_counts`. Ensure calls with the same profiled shape but different
eligibility, such as 2047 and 2048 tokens, use separate cache entries.
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Review comments at @tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py:
- Around line 194-199: Restore the default N=256 eligibility guard in the
runner’s `get_valid_tactics` flow: allow N=256 only for at least 2048 raw
tokens, when all experts are local and expert-count scheduling is disabled.
Capture `token_final_scales.size(0)` before autotuning and use it for
eligibility; do not infer raw tokens from profiled inputs. Include eligibility
in the default runner identity to prevent cached N=256 tactics from crossing
eligible and ineligible calls. Preserve explicit override behavior.
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…when the override is unset The FC2 finalize-fusion runner's unset default pinned the N tile to 128, which removed N256 from the autotuner's candidate set for every shape. Prefill-sized FC2 calls and attention-TP layouts profile faster at N256 and lost it. Leave TRTLLM_CUTEDSL_FC2_N_TILE_SIZE unset to autotune over both tiles, as before the override existed. Setting it to 128 or 256 still pins that tile, and 0 still autotunes. Signed-off-by: chenfeiz0326 <203214996+chenfeiz0326@users.noreply.github.com>
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The only failure in L0 #64222 is That is the known flake in #16281. The same test failed on 12 other PRs in L0 #64000-64245, always at about 0.05% and with different parameters each time. It exercises the FC1 gather/activation-fusion runner, which never reads the FC2 N-tile setting this PR changes. I've re-run CI on the same commit, so the stages that passed are reused. |
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Description
Step 2 of the three-step decline in NVBug 6919819 (GB200
ctx_onlyqwen3-235b-fp4 8k1kcon1024/con64/con1). Step 1 is fixed by #20250; step 3 (the NGC 26.09 image) is #20252.
This PR also fixes NVBug 6928214, the same regression on CuTeDSL attention-TP layouts.
Root cause
#17015 added
TRTLLM_CUTEDSL_FC2_N_TILE_SIZEand, when the variable is unset, pinned theCuTeDSL FC2 finalize-fusion GEMM (
cute_dsl_nvfp4_grouped_gemm_finalize_inplace_blackwell)to N=128. Before it, the autotuner chose between N=128 and N=256 for every shape. The pin
removed N=256 from the candidates everywhere, so every shape where N=256 wins got slower:
ctx_onlycases steps down at [None][perf] Add CUTEDSL FC2 N-tile tuning override #17015's build:con1024 -1.75%, con64 -1.40% and con1 -2.55% Output Token Throughput. These are CI medians of
b2950-2967 (without [None][perf] Add CUTEDSL FC2 N-tile tuning override #17015) vs b2968-2990 (with it). The per-rep autotuner caches show
N=256 winning in the
tile_m=256buckets, 11-19% faster per call in the prefill-sized ones.ws4/ws8/ws16, 128-4096 tokens, are up to 1.20x slower in the FC2 kernel. A same-build A/B
with
TRTLLM_CUTEDSL_FC2_N_TILE_SIZE=0restores the pre-[None][perf] Add CUTEDSL FC2 N-tile tuning override #17015 numbers in every case.The change
Unset
TRTLLM_CUTEDSL_FC2_N_TILE_SIZEautotunes over N=128 and N=256 again, as before #17015.The variable stays as an explicit opt-in.
128or256pins that tile for deployments whereit is known to win end to end.
0autotunes, same as unset.An earlier revision of this PR kept the pin and reopened N=256 only for large prefill calls.
Review pointed out that this left the 6928214 shapes regressed. My earlier claim that the pin
carried the large DSR1 decode gains in that CI window also did not hold up per build (see the
review thread). Restoring autotune is the simpler fix and covers both bugs.
Perf verification
GB200 (oci-hsg), the post-merge CI wheel of
fca08032f7(GenPostMergeBuilds 2911) in itspinned image (NGC PyTorch 26.09), CI harness from the same commit.
cute_dsl_custom_ops.pyisbyte-identical between that commit and this PR's base. The "autotune" arm is the stock wheel with
TRTLLM_CUTEDSL_FC2_N_TILE_SIZE=0, which is exactly what unset does with this PR. Everycomparison is same-node, inside one allocation.
DeepSeek-R1 NVFP4 WideEP decode, the regime #17015 tuned:
bench_moeDEP4 (attention DP + MoE EP=4),CuTeDSL, CUDA graph, 12 timed iterations, pin and autotune interleaved twice on one GB200 node:
Autotune matches the pin within rep-to-rep spread at decode sizes and is faster at 4096 tokens.
#17015's 1.2-1.8% gain on DSR1 WideEP EP32 (B200) is still available by setting
=128.All three steps together: #20250, the FC2 autotuning this PR restores, and the 26.08 compat
driver from #20252 (bind-mounted). The arm ran the earlier, scoped revision of this PR, which
makes the same FC2 choices on qwen3 as autotuning does in the buckets that matter. Same node,
Output Token Throughput:
Test Coverage
tests/unittest/_torch/moe/fused_moe/test_cutedsl_fc2_tuning.py(listed inl0_b200.yml):test_fc2_n_tile_size_override[None-...]now expects the unset variable to offer both Ntiles, with no override in the runner identity. The
0,128and256cases are unchanged.30dc23ffe58, whosecute_dsl_custom_ops.pyand test file are byte-identical to this PR'sbase:
PR Checklist
🤖 Generated with Claude Code
Dev Engineer Review
cute_dsl_custom_ops.py. It does not change FC2 tile selection or other runtime behavior.QA Engineer Review
No test changes.
Per-File QA Perspective
tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py— Import formatting changes only. No new behavior requires QA verification.