The collide and stream passes each moved the whole population field
through two buffers (304 bytes per cell update). The AA pattern (Bailey
et al. 2009; Wittmann et al. 2013) keeps one buffer: even steps collide
each cell in place with its directions swapped, odd steps read a cell's
incoming populations at its neighbours and write the outgoing ones where
the next step reads them, so each thread writes the addresses it read
and streaming happens inside the collision (152 bytes). A cell's
coordinates are computed once and neighbours wrap with a compare and an
add. The per-link wall rules gather their sources before a scatter
writes them, so no link overwrites another's post-collision value, and
the momentum exchange is taken in the gather of the step whose forces
are read back.
Options::streaming picks the layout; Auto, what a solve uses, streams
in place, which measured faster on every device tried: 1.5x on an RTX
4090, 1.85x on the GB10 and 1.45x on an M4 Max at periodic 128^3.
Options::profile times each kernel with timestamp queries for the
throughput test, which now runs both layouts. The verification tests
run in both layouts, with two new ones comparing fields, mass, forces
and heat with the CPU after odd and even step counts across submission
boundaries.
Part of #562.
A new L7 crate: the CPU scheme's collision (TRT with Guo forcing), the
per-link streaming rules (bounce-back with Ladd's term, slip reflection,
open placeholders), the Zou–He boundaries and the momentum exchange per
force group as wgpu compute shaders, stepping the flow fieldcraft_lbm's
Flow prepared, through its Backend trait: populations stay on the device,
a chunk of steps is one submission, and only frames, forces and the
stability check come back. Populations are stored as deviations from
their weights, which keeps single precision honest: the verification
cases (Poiseuille with Λ = 3/16, Couette with wall forces, Taylor–Green,
a sphere's drag, a Zou–He channel) agree with the CPU to 2–5e-6 in
velocity, 1e-6 in density and 5e-5 in force, with those tolerances stated
in the tests. The rules are applied one thread per link rather than in a
per-cell loop, which lavapipe's JIT miscompiles (and which diverges on
real GPUs anyway). Native only for now; the browser registers nothing.
Part of #529.
Christian Svensson