Hadoop does not scale as expected
---------------------------------
Key: HDFS-2091
URL: https://issues.apache.org/jira/browse/HDFS-2091
Project: Hadoop HDFS
Issue Type: Bug
Environment: Linux, 8 nodes.
Reporter: Alberto Andreotti
The more nodes I add to this application, the slower it goes. This is the app's
map,
public void map(IntWritable linearPos, FloatWritable heat, Context context
) throws IOException, InterruptedException {
int myLinearPos = linearPos.get();
//Distribute my value to the previous and the next
linearPos.set(myLinearPos - 1);
context.write(linearPos, heat);
linearPos.set(myLinearPos + 1);
context.write(linearPos, heat);
//Distribute my value to the cells above and below
linearPos.set(myLinearPos - MatrixData.Length());
context.write(linearPos, heat);
linearPos.set(myLinearPos + MatrixData.Length());
context.write(linearPos, heat);
}//end map
and this is the reduce,
public void reduce(IntWritable linearPos, Iterable<FloatWritable> fwValues,
Context context) throws IOException, InterruptedException {
//Handle first and last "cold" boundaries
if(linearPos.get()<0 || linearPos.get()>MatrixData.LinearSize()){
return;
}
if(linearPos.get()==MatrixData.HeatSourceLinearPos()){
context.write(linearPos, new
FloatWritable(MatrixData.HeatSourceTemperature()));
return;
}
float result = 0.0f;
//Add all the values
for(FloatWritable heat : fwValues) {
result += heat.get();
}
context.write(linearPos, new FloatWritable(result/4) );
}
For example, with 6 nodes I get a running time of 15minutes, and with 4 nodes I
get a running time of 8minutes!.
This is how I generated the input,
public static void main(String[] args) throws IOException {
//Write file in the local dir
String uri = "/home/beto/mySeq";
Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(URI.create(uri), conf);
Path path = new Path(uri);
IntWritable key = new IntWritable();
FloatWritable value = new FloatWritable(0.0f);
SequenceFile.Writer writer = null;
try {
writer = SequenceFile.createWriter(fs, conf, path, key.getClass(),
value.getClass());
int step = MatrixData.LinearSize()/10;
int limit = step;
for (int i = 0; i <= MatrixData.LinearSize(); i++) {
key.set(i);
if(i>limit){
System.out.println("*");
limit +=step;
}
if(i==MatrixData.HeatSourceLinearPos()) {
writer.append(key, new
FloatWritable(MatrixData.HeatSourceTemperature()));
continue;
}
writer.append(key, value);
}
} finally {
IOUtils.closeStream(writer);
}
}
I'm basically solving a heat transfer problem in a squared section. Pretty
simple. The input data is being stored as a (key, value) pairs, read in this
way, processed, and written again in the same format.
Any thoughts?
Alberto.
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