ACT_ADVANCE
if (bp->b_act_count > ACT_MAX - ACT_ADVANCE)
bp->b_act_count += ACT_ADVANCE;
m->act_count += (actcount + ACT_ADVANCE);
m->act_count += (actcount + ACT_ADVANCE + 1);
m->act_count += ACT_ADVANCE + actcount;
m->act_count += ACT_ADVANCE + actcount;