35 lines
2.3 KiB
Markdown
35 lines
2.3 KiB
Markdown
# MRT reuse in blocked/refined stepping (candidate)
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Base `9805383`. Opt-in by `LBMConfig(collision="MRT")`; SRT remains the default and its
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kernel statements, including the shifted retained-delta/direct-momentum path, are unchanged
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under the alternate compile branch.
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## What changes
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- `blocks.cu` (`BlockStep`), `step/block_channel.cu` (`BlockChannelStep`) and `refinement.cu`
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(`patch_collide`) gain a compile-time `COLLISION_MODEL == 2` branch that repeats the uniform
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`collide_dispatch` sequence with zero forcing: `compute_rho_u`, omega clamp to
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`[OMEGA_COLLISION_MIN, OMEGA_COLLISION_MAX]`, `collide_mrt_no_force`.
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- Kernel and host guards accept SRT or MRT only. TRT, LES, non-FP32 storage, non-double-buffer
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streaming, and shifted dense/mixed refinement remain rejected.
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- Transfers, subcycling, BFL, raw exchange ledgers and channel closures are untouched. The
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existing pressure-NEQ outlet selects its MRT branch (unknowns-only NEQ extrapolation) from
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`COLLISION_MODEL`, so MRT channel parity must use uniform MRT as reference.
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## Contracts (existing operator, not redefined here)
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- Shifted storage uses the existing MRT representation (equilibrium moments from `rho - 1`),
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not the SRT retained-delta/direct-momentum precision. Mass and momentum are conserved
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moments (relaxation rate 0).
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- Kinetic rates are the operator's fixed `s_e = s_eps = s_q = 1.2` on every level; the shear
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rate is the existing per-level omega. Under acoustic 2:1 scaling the physical bulk viscosity
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therefore halves on each finer level. No rate rescaling is applied.
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- No LES, no external forcing (`d_params` force fields stay zero).
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- Shifted Jets stay SRT-only: `body/jets.py` keeps its existing guard. This extension does not
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provide shifted MRT Jets.
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- `PeriodicBlockStepper` (uniform blocked stepper) remained SRT-only; it was removed in 8e1feff and its single-level capability, still SRT-only, is `FixedPatchStepper(refinements=[])`.
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## Evidence status
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CPU: `tests/unit/test_refinement_mrt.py` (host guards, source contract). GPU:
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`tests/validation/multigrid/test_refinement_mrt_gpu.py` (uniform-MRT channel parity, AMR MRT
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smoke) and unchanged SRT GPU tests. Strict parity may be subject to cross-kernel FMA
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contraction differences already documented for SRT. Not a DFG/benchmark validation.
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