spark.shuffle.registration.timeout: 5000: Timeout in milliseconds for registration to the external shuffle service. In order to install Hadoop, we need java first so first, we install java in our Ubuntu. / mapreduce / src / java / mapred-default.xml. Add comment. Reply. When the in-memory buffer reaches a threshold size which is controlled by mapreduce.reduce.shuffle.merge.percent or reaches a threshold number of map outputs which is controlled by mapreduce.reduce.merge.inmem.threshold, it is merged and spilled to disk. xml version = "1.0"?>