Documentation says, map tasks are run in DataNodes and have data locality constraints which the scheduler tries to honor and reduce tasks can run anywhere in the cluster. The statement "can run anywhere in the cluster" for reduce tasks, is referring to only DataNodes in the cluster OR is ResourceManager machine also considered part of the cluster, so that Reduce tasks are allowed to run in ResourceManager also ??
"can run anywhere in the cluster" - Meaning anywhere where nodemanagers have been deployed, it can be any slave node or master node if nodemanager is installed on master nodes.
Hope this information helps!
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