BDR jobs running with few mappers

[#1]

I am upgrading from CDH version 5.4 to 5.7.
Can some one post the improvements that I can expect in BDR (backup and data recovery) from CDH 5.4 tp CDH 5.7?
Even though I set 40 mappers for a bdr job it runs on 40 for a while and then it will reduce the mappers to say 20 or 15 and runs for several hours. Because of this we are not able to keep up with the replication. Is this fixed in CDH 5.7?
Need to know what improvements are done for BDR on 5.7.

[#2]

The # of mappers in BDR is definitively determined by the passed configuration. Could you share JSONs or Screenshots of your configured job(s) illustrating the problem?