protected function serializeToStorage()
  {
    $this->debugLog('serializeToStorage()');
    $this->storage->updateLists($this->lists);
    $this->storage->setUpdaterState($this->next_request_timestamp, $this->error_count);
    
    $this->debugLog('current memory usage: '.memory_get_usage());
    $this->debugLog('peak memory usage   : '.memory_get_peak_usage());
  }