October 17, 2010

Apache Hama: BSP based Pi Estimator

Pi Estimator

The value of PI can be calculated in a number of ways. Consider the following method of estimating PI

  • Inscribe a circle in a square
  • Randomly generate points in the square
  • Determine the number of points in the square that are also in the circle
  • Let r be the number of points in the circle divided by the number of points in the square
  • PI ~ 4 r

Serial pseudo code for this procedure as below:
iterations = 10000
circle_count = 0

do j = 1,iterations
  generate 2 random numbers between 0 and 1
  xcoordinate = random1
  ycoordinate = random2
  if (xcoordinate, ycoordinate) inside circle
  then circle_count = circle_count + 1
end do

PI = 4.0*circle_count/iterations

The BSP implementation for Pi

A distributed strategy in HAMA with BSP programming model, is break the loop into portions that can be executed by the tasks.

  • Each task executes locally its portion of the loop a number of times.
  • One task acts as master and collects the results through the BSP communication interface.

public class PiEstimator {
  private static String MASTER_TASK = "master.task.";

  public static class MyEstimator extends BSP {
    public static final Log LOG = LogFactory.getLog(MyEstimator.class);
    private Configuration conf;
    private String masterTask;
    private static final int iterations = 10000;

    @Override
    public void bsp(BSPPeer bspPeer) throws IOException, KeeperException,
        InterruptedException {
      int in = 0, out = 0;
      for (int i = 0; i < iterations; i++) {
        double x = 2.0 * Math.random() - 1.0, y = 2.0 * Math.random() - 1.0;
        if ((Math.sqrt(x * x + y * y) < 1.0)) {
          in++;
        } else {
          out++;
        }
      }

      byte[] tagName = Bytes.toBytes(getName().toString());
      byte[] myData = Bytes.toBytes(4.0 * (double) in / (double) iterations);
      BSPMessage estimate = new BSPMessage(tagName, myData);

      bspPeer.send(bspPeer.getAddress(masterTask), estimate);
      bspPeer.sync();

      double pi = 0.0;
      BSPMessage received;
      while ((received = bspPeer.getCurrentMessage()) != null) {
        LOG.info("Receives messages:" + Bytes.toDouble(received.getData()));
        if(pi == 0.0) {
          pi = Bytes.toDouble(received.getData());
        } else {
          pi = (pi + Bytes.toDouble(received.getData())) / 2;
        }
      }

      if (pi != 0.0) {
        LOG.info("\nEstimated value of PI is " + pi);
      }
    }

    @Override
    public Configuration getConf() {
      return conf;
    }

    @Override
    public void setConf(Configuration conf) {
      this.conf = conf;
      this.masterTask = conf.get(MASTER_TASK);
    }

  }

  public static void main(String[] args) throws InterruptedException,
      IOException {
    // BSP job configuration
    HamaConfiguration conf = new HamaConfiguration();
    // Execute locally
    // conf.set("bsp.master.address", "local");

    BSPJob bsp = new BSPJob(conf, PiEstimator.class);
    // Set the job name
    bsp.setJobName("pi estimation example");
    bsp.setBspClass(MyEstimator.class);

    BSPJobClient jobClient = new BSPJobClient(conf);
    ClusterStatus cluster = jobClient.getClusterStatus(true);
    // Choose one as a master
    for (String name : cluster.getActiveGroomNames()) {
      conf.set(MASTER_TASK, name);
      break;
    }

    BSPJobClient.runJob(bsp);
  }
}

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