[https://nvbugs/6633931][fix] Size the SM90 fp8 block-scale MoE workspace for the buffers it actually uses - #18323
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…the buffers it uses Under fp8 activation fusion the inputs buffer holds pre-quantized fp8 activations plus their padded 1x128 scales, not the bf16 activations permuted_data_size is sized for, so max()-ing with the bf16 sizes keeps it at twice what is needed. Only the fused path writes FC2 output into the outputs buffer, so the unfused path no longer sizes for it either. DeepSeek-V3.2 at 65536 MoE rows: 14.068 -> 10.679 GiB, outputs bit-identical. Signed-off-by: Pranav Shrestha <254760092+pranav-nvidia@users.noreply.github.com>
…rkspace Signed-off-by: Pranav Shrestha <254760092+pranav-nvidia@users.noreply.github.com>
…zing comments Signed-off-by: Pranav Shrestha <254760092+pranav-nvidia@users.noreply.github.com>
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WalkthroughDeepSeek FP8 block-scale workspace sizing now separates prequantized and unfused allocation paths. Prequantized paths include packed activation scale regions and larger FC1/FC2 output storage. Unfused paths reserve additional FC2 output capacity. ChangesDeepSeek FP8 workspace allocation
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: ⚪ Minimal · up to The PR reduces SM90 FP8 MoE workspace allocation without changing kernels or launch parameters. No actionable merge-blocking risk remains; the outstanding const-qualification cleanup is routine. 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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In `@cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu`:
- Line 3338: In the DeepSeek MoE GEMM setup, declare blockscale_gemm_runner and
the deepseek_fc1_workspace_size and deepseek_fc2_workspace_size locals as const
using east-const style, without changing their initialization or subsequent
read-only use.
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cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu
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4 participants
Dev Engineer Review
fc1_output.QA Engineer Review
No test changes.
Description
On SM90,
getWorkspaceDeviceBufferSizesover-sizes the two overlapped MoE buffers for the fp8 block-scale path, and #16849 made it worse.Under fp8 activation fusion (
useFp8BlockScaleActFusion(), SM90 only) the overlapped inputs buffer holds pre-quantized fp8 activations plus their padded 1x128 scales — never the bf16 activations thatpermuted_data_sizeandfc1_result_sizeare sized for. #16849 already computes the correct size withfp8BlockScaleRegionBytes(...), but takes amax()with the stale bf16 sizes instead of replacing them. Sincesizeof(T) == sizeof(UnfusedGemmOutputType) == 2,permuted_data_sizealone pins the buffer atpermuted_elems * 2, roughly twice what the fused path needs. This replaces rather than maxes.The FC2 GEMM output is now reserved in exactly one of the two buffers, selected by the same predicate the forward pass uses to route it (
isActivationPrequantized()/use_fused_block_scale_quant): the fused path writes it into the outputs buffer (glu_inter_result_, because its inputfc1_result_is aliased ontofc2_result_), the unfused path writesfc2_result_in the inputs buffer. It was previously counted in both. This also restoresoutputs = fc1_outputon the unfused path — #16849 widened it tomax(fc1, fc2)unconditionally, so non-SM90 architectures were reserving an FC2-output-sized buffer they never write to.Measured on H100 (SM90) at DeepSeek-V3.2 dimensions (hidden 7168, inter 2048, 256 experts, top_k 8), sweeping MoE rows, reading the workspace size the op logs at DEBUG:
At 65536 rows that is 15,105,925,632 -> 11,467,047,424 bytes, a 24.1% reduction. This is sizing only; no kernel or launch parameter changes.
Context: the 14.02 GiB allocation is what OOMs in NVBug 6633931 on 8xH200. That test also runs MoE unchunked, which is fixed separately in #18322; the two changes are independent and both are needed to give the test real margin.
Test Coverage
tests/unittest/_torch/modules/moe/test_moe_backend.py -k "FP8_BLOCK_SCALES and CUTLASS"— 10 passed on H100, run on the unpatched build as a control.Because that suite only covers small shapes (seq 1-8, hidden 256-2048) where the workspace is negligible, an under-sized buffer that only overflows at large
num_moe_inputswould corrupt FC2's input silently rather than crash. So the change was additionally checked at full DeepSeek-V3.2 dimensions up to 65536 MoE rows (524288 expanded rows) with identical seeds: output fingerprints are bit-identical before and after at every row count (out_sum194420032929792.0,out_absmax14483456.0,out_mean413869.1875, 0 NaN, 0 Inf), andgetWorkspaceSizeno longer needs the removed capacity.PR Checklist
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