amazon web services - How to set the precise max number of concurrently running tasks per node in Hadoop 2.4.0 on Elastic MapReduce

Question

According to http://blog.cloudera.com/blog/2014/04/apache-hadoop-yarn-avoiding-6-time-consuming-gotchas/, the formula for determining the number of concurrently running tasks per node is:

min (yarn.nodemanager.resource.memory-mb / mapreduce.[map|reduce].memory.mb, 
     yarn.nodemanager.resource.cpu-vcores / mapreduce.[map|reduce].cpu.vcores) .

However, on setting these parameters to (for a cluster of c3.2xlarges):

yarn.nodemanager.resource.memory-mb = 14336

mapreduce.map.memory.mb = 2048

yarn.nodemanager.resource.cpu-vcores = 8

mapreduce.map.cpu.vcores = 1,

I find I'm only getting up to 4 tasks running concurrently per node when the formula says 7 should be. What's the deal?

I'm running Hadoop 2.4.0 on AMI 3.1.0.

Answer 1

My empirical formula was incorrect. The formula provided by Cloudera is the correct one and appears to give the expected number of concurrently running tasks, at least on AMI 3.3.1.