I want to fix a limit for the number of running mapper per mapreduce job, I set the following parameter in my mapred-site.xml file
But it seems that this parameter has no effect. I found a job which takes 44543 mappers and in the configuration file i found the right parameter with the rigth value (25000).
Is there another parameter to activate this limit?
No of mappers depends on input split size it cannot be manually configured. Input split is a logical representation of data and block is a physical representation of data. An input split indicates the start and end point of data in a block.
There is one mapper per input split. so the number of mappers is not completely dependent on the number blocks. Depending upon the configured size, the number of splits varies. There might be one split per block, one split per two blocks or maybe two splits per block and so on.