How Tez decides the Reducers order and spins the containers

[#1]

In My HDP 3.1.5 cluster, Tez is not following the reducer order in the VERTICES (shown in the first pic). I am interested in knowing how Tez decides the order and number of containers in a VERTICES. I can even see single-digit containers in a few VERTICES. Resources are free on the cluster but it has very few containers running. 

[#2]

You can check the explain plan . It provided the order of the execution.

It is not a rule that certain reucers would start after all the mapper phase.

The ceratin mapper phase depends on the results from the reducers too. You can check the explain plan .

If you want map join then you can set property map.auto.conevrt.join =true