autovacuum_multixact_freeze_max_age

The multixact counterpart of autovacuum_freeze_max_age, default 400M: at this multixact age a forced aggressive vacuum runs to freeze old multixact references. Vacuum also triggers on member-space usage: once the member area passes half its capacity, the effective threshold drops automatically. Tune it with the same caution as the xid version, and remember that member space, not age, is the limit that fills silently.