nutch 生产者队列的大小如何控制 threadcount * 50

如果topN 设置为1000万 ，不会这1000万都放到QueueFeeder（内存）中，而是从文件系统中（hdfs）中迭代不断填充QueueFeeder。
队列中默认存放 threadcount * 50 。

    这个类的作用是从文件系统读文件填充队列。

/**
   * This class feeds the queues with input items, and re-fills them as
   * items are consumed by FetcherThread-s.
   */
  private static class QueueFeeder extends Thread {
    private final Context context;
    private final FetchItemQueues queues;
    private final int size;
    private Iterator<FetchEntry> currentIter;  //FetchEntry实现了 org.apache.hadoop.io.Writable
    boolean hasMore;
    private long timelimit = -1;

    public QueueFeeder(Context context,
        FetchItemQueues queues, int size)
    throws IOException, InterruptedException {
      this.context = context;
      this.queues = queues;
      this.size = size;
      this.setDaemon(true);
      this.setName("QueueFeeder");
      hasMore = context.nextKey();
      if (hasMore) {
        currentIter = context.getValues().iterator();
      }
      // the value of the time limit is either -1 or the time where it should finish
      timelimit = context.getConfiguration().getLong("fetcher.timelimit", -1); 
    }

    @Override
    public void run() {
      int cnt = 0;
      int timelimitcount = 0;
      try {
        while (hasMore) {
          if (System.currentTimeMillis() >= timelimit && timelimit != -1) {
            // enough .. lets' simply
            // read all the entries from the input without processing them
            while (currentIter.hasNext()) {
              currentIter.next();
              timelimitcount++;
            }
            hasMore = context.nextKey();
            if (hasMore) {
              currentIter = context.getValues().iterator();
            }
            continue;
          }
          int feed = size - queues.getTotalSize();
          if (feed <= 0) {
            // queues are full - spin-wait until they have some free space
            try {
              Thread.sleep(1000);
            } catch (final Exception e) {};
            continue;
          }
          if (LOG.isDebugEnabled()) {
            LOG.debug("-feeding " + feed + " input urls ...");
          }
          while (feed > 0 && currentIter.hasNext()) {
            FetchEntry entry = currentIter.next();
            final String url =
              TableUtil.unreverseUrl(entry.getKey());
            queues.addFetchItem(url, entry.getWebPage());
            feed--;
            cnt++;
          }
          if (currentIter.hasNext()) {
            continue; // finish items in current list before reading next key
          }
          hasMore = context.nextKey();
          if (hasMore) {
            currentIter = context.getValues().iterator();
          }
        }
      } catch (Exception e) {
        LOG.error("QueueFeeder error reading input, record " + cnt, e);
        return;
      }
      LOG.info("QueueFeeder finished: total " + cnt + " records. Hit by time limit :"
          + timelimitcount);
      context.getCounter("FetcherStatus","HitByTimeLimit-QueueFeeder").increment(timelimitcount);
    }
  }