Take the 2-minute tour ×  If I don't specify the number of
mappers, how would the number be determined? Is there a default
setting read from a configuration file (such as mapred-
site.xml)?  share|improve this question add comment  2 Answers
up vote 3 down vote accepted  Adding more to what Chris added
above:    1. The number of maps is usually driven by the number
of DFS blocks in the input files. Although that causes people to
adjust their DFS block size to adjust the number of maps.    2.
The right level of parallelism for maps seems to be around
10-100 maps/node, although this can go upto 300 or so for very
cpu-light map tasks. Task setup takes awhile, so it is best if
the maps take at least a minute to execute.    3. You can
increased number of Map task by modifying JobConf's
conf.setNumMapTasks(int num). Note: This could increase the
number of map tasks, but will not set the number below that
which Hadoop determines via splitting the input data.  Finally
controlling the number of maps is subtle. The map