how many mappers and reduces will get created for

Question

I am always confused on how many mappers and reduces will get created for a particular task in hive. e.g If block size = 128mb and there are 365 files each maps to a date in a year(file size=1 mb each). There is partition based on date column. In this case how many mappers and reducers will be run during loading the data?

Answer 1

Mappers:

Number of mappers depends on various factors such as how the data is distributed among nodes, input format, execution engine and configuration params. See also here: https://cwiki.apache.org/confluence/display/TEZ/How+initial+task+parallelism+works

MR uses CombineInputFormat, while Tez uses grouped splits.

Tez:

set tez.grouping.min-size=16777216; -- 16 MB min split
set tez.grouping.max-size=1073741824; -- 1 GB max split

MapReduce:

set mapreduce.input.fileinputformat.split.minsize=16777216; -- 16 MB
set mapreduce.input.fileinputformat.split.minsize=1073741824; -- 1 GB

Also Mappers are running on data nodes where the data is located, that is why manually controlling the number of mappers is not an easy task, not always possible to combine input.

Reducers: Controlling the number of reducers is much easier. The number of reducers determined according to

mapreduce.job.reduces - The default number of reduce tasks per job. Typically set to a prime close to the number of available hosts. Ignored when mapred.job.tracker is "local". Hadoop set this to 1 by default, whereas Hive uses -1 as its default value. By setting this property to -1, Hive will automatically figure out what should be the number of reducers.

hive.exec.reducers.bytes.per.reducer - The default in Hive 0.14.0 and earlier is 1 GB.

Also hive.exec.reducers.max - Maximum number of reducers that will be used. If mapreduce.job.reduces is negative, Hive will use this as the maximum number of reducers when automatically determining the number of reducers.

So, if you want to increase reducers parallelism, increase hive.exec.reducers.max and decrease hive.exec.reducers.bytes.per.reducer