title: MapReduce: Increase number of concurrent mapper tasks
body: <p>I am using AWS EMR to run a map reduce job. My input set contains 1 million files each of around 15KB. Since input files are very small, so this will lead to a huge number of mappers. So, I changed s3 block size to 20KB and used 5 r3.2xlarge instances but number of concurrent tasks running is still just 30. Shouldn't the job run more number of concurrent mappers now after reducing the block size or even after reducing block size, memory taken by each mapper is still same? </p>
  
  <p>How can I limit the memory usage of each mapper or increase the number of concurrent mapper tasks? The current expected completion time is 100hours, will combining these files to lesser number of bigger files, like 400MB files, increase the processing time? </p>