public static void checkJobTracker(JobConf conf, Logger LOG) {

    try {
      byte[] buffer = new byte[1024];
      int threshold = conf.getInt("mapred.throttle.threshold.percent", DEFAULT_MEMORY_GC_PERCENT);
      int retry = conf.getInt("mapred.throttle.retry.period", DEFAULT_RETRY_PERIOD);

      // If the threshold is 100 percent, then there is no throttling
      if (threshold == 100) {
        return;
      }

      // This is the Job Tracker URL
      String tracker = JobTrackerURLResolver.getURL(conf) + "/gc.jsp?threshold=" + threshold;

      while (true) {
        // read in the first 1K characters from the URL
        URL url = new URL(tracker);
        LOG.debug("Throttle: URL " + tracker);
        InputStream in = null;
        try {
          in = url.openStream();
          in.read(buffer);
          in.close();
          in = null;
        } finally {
          IOUtils.closeStream(in);
        }
        String fetchString = new String(buffer);

        // fetch the xml tag <dogc>xxx</dogc>
        Pattern dowait = Pattern.compile("<dogc>", Pattern.CASE_INSENSITIVE
                                         | Pattern.DOTALL | Pattern.MULTILINE);
        String[] results = dowait.split(fetchString);
        if (results.length != 2) {
          throw new IOException("Throttle: Unable to parse response of URL "
                                + url + ". Get retuned " + fetchString);
        }
        dowait = Pattern.compile("</dogc>", Pattern.CASE_INSENSITIVE
                                 | Pattern.DOTALL | Pattern.MULTILINE);
        results = dowait.split(results[1]);
        if (results.length < 1) {
          throw new IOException("Throttle: Unable to parse response of URL "
                                + url + ". Get retuned " + fetchString);
        }

        // if the jobtracker signalled that the threshold is not exceeded,
        // then we return immediately.
        if (results[0].trim().compareToIgnoreCase("false") == 0) {
          return;
        }

        // The JobTracker has exceeded its threshold and is doing a GC.
        // The client has to wait and retry.
        LOG.warn("Job is being throttled because of resource crunch on the "
                 + "JobTracker. Will retry in " + retry + " seconds..");
        Thread.sleep(retry * 1000L);
      }
    } catch (Exception e) {
      LOG.warn("Job is not being throttled. " + e);
    }
  }
Fetch http://tracker.om:/gc.jsp?threshold=period.
ql/src/java/org/apache/hadoop/hive/ql/exec/mr/Throttle.java