Dust Time Integration

CosmicGrain does not simply call every dust process on every gravity step. The current integration layer in src/main/run.cc accumulates the physical time covered by synchronization points and performs a dust update when the configured cadence is reached.

Cadence state

DustUpdateCadence is a runtime integer parameter. The current source accepts values from 1 through 1,000,000. A value of 1 updates the dust physics at every eligible synchronization point; larger values accumulate multiple intervals before calling the dust update.

For cosmological integration, the code converts the normal GADGET integration interval to physical time using the current cosmological factor before adding it to DustAccumulatedPhysicalTime.

When the cadence is reached, advance_dust_physics() calls:

update_dust_dynamics(..., DustAccumulatedPhysicalTime, ...)
dust_global_synchronization(...)

and then clears the accumulated interval and per-window creation/destruction bookkeeping.

Output-safe flushing

The integration interface can force an update before the nominal cadence is reached. This is important at output or other synchronization boundaries: a snapshot should not represent particle state at one time while silently leaving a physically elapsed dust interval unapplied.

Writing a new process

A new dust process should use the physical interval passed through the dust update rather than infer a timestep from an unrelated particle time bin. Where a process represents a continuous rate, prefer a time integration that remains well behaved when the cadence is changed. Changes to cadence-sensitive physics must be tested with more than one DustUpdateCadence value and across a restart boundary.

Restart interaction

The accumulated synchronization-point count and physical interval are persisted in the CosmicGrain restart extension. See Restart Files. The active DustUpdateCadence value itself is read from the parameter file on startup.