POSDIFF
ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
limit += POSDIFF(completed, dql->prev_num_queued) +
slack = POSDIFF(limit + dql->prev_ovlimit,
POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
limit = POSDIFF(limit, dql->lowest_slack);