Performance

• At 2048³, the dust population grows to ~15M superparticles by z~3. To
• avoid these dominating the gravity tree, newly spawned dust is assigned a gravity timebin of max(DUST_MIN_TIMEBIN, HighestActiveTimeBin), ensuring it lands on a bin that is synchronized with the current hierarchy. At late times HighestActiveTimeBin ≤ 15 so DUST_MIN_TIMEBIN dominates; at early times (z > 10) HighestActiveTimeBin can be 21+, and without the clamp dust would spawn on an unsynchronized bin causing a collective gravity hang on multi-node runs. *
• IMPORTANT: spawn_dust_particle caps the timebin at birth, but Gadget's
• ongoing timestep criterion (timestep.cc: get_timestep_grav) can migrate
• dust back to short bins after the first gravity force evaluation.
• get_timestep_grav() in timestep.cc returns TIMEBASE-1 for all dust particles,
• causing timebins_get_bin_and_do_validity_checks to assign the highest
• currently-synchronized bin. Dust migrates upward naturally from DUST_MIN_TIMEBIN
• within a few sync-points after spawning. *
• Dust physics (drag, growth, sputtering, etc.) runs every 10 gravity steps
• via the cadence guard in update_dust_dynamics(), with dt scaled by 10
• to compensate. All routines that receive this scaled dt use the exact
• analytical form (1 − exp(−dt/τ)) rather than linear approximations,
• so the result is independent of step size as long as τ is well resolved.