Flink Batch job: All slots for groupReduce task scheduled on same machine

[Aneesha Kaushal]

Hello, 

I have a fink batch job, where I am grouping dataset on some keys, and then 
using group reduce. Parallelism is set to 16. 
The slots for the Map task is distributed across all the machines, but for 
GroupReduce all the slots are being assigned to the same machine. Can you help 
me understand why/when this can happen? 
Code looks something like: 
dataset.map(MapFunction())
  .groupBy(<keys to groupon>)
  .sortGroup(<key to sort on>, Order.DESCENDING)
  .reduceGroup(GroupReduceFunction()).name("Group reduce")
From flink dashboard: 




Thanks in advance
Aneesha