When running SimpleShortesPath example, it completed well if the number of worker is 1.

hadoop jar \
/home/wb/giraph-1.0.0/giraph-examples/target/giraph-examples-1.0.0-for-hadoop-0.20.203.0-jar-with-dependencies.jar \
org.apache.giraph.GiraphRunner \
org.apache.giraph.examples.SimpleShortestPathsVertex \
-vif org.apache.giraph.io.formats.JsonLongDoubleFloatDoubleVertexInputFormat \
-vip /in/input \
-of org.apache.giraph.io.formats.IdWithValueTextOutputFormat \
-op /outShortest \
-w 1


However, when I increase the number of workers , like 2, map tasks were killed.

hadoop jar \
/home/wb/giraph-1.0.0/giraph-examples/target/giraph-examples-1.0.0-for-hadoop-0.20.203.0-jar-with-dependencies.jar \
org.apache.giraph.GiraphRunner \
org.apache.giraph.examples.SimpleShortestPathsVertex \
-vif org.apache.giraph.io.formats.JsonLongDoubleFloatDoubleVertexInputFormat \
-vip /in/input \
-of org.apache.giraph.io.formats.IdWithValueTextOutputFormat \
-op /outShortest \
-w 2


How can I increase the number of workers?