  epoch_t get_min_pg_epoch() {
    Mutex::Locker l(pg_epoch_lock);
    if (pg_epochs.empty())
      return 0;
    else
      return *pg_epochs.begin();
  }
  epoch_t get_min_pg_epoch() {
    Mutex::Locker l(pg_epoch_lock);
    if (pg_epochs.empty())
      return 0;
    else
      return *pg_epochs.begin();
  }