[openstack] how to speed up live migration?

Hello All, I am looking for a solution to speed up live migration. Instances where ram is used heavily like java application servers, live migration take a long time (more than 20 minutes for 8GB ram instance) and converge mode is already set to True in nova.conf. I also tried with post_copy but it does not change. After the first live migration (very solow) if I try to migrate again it is very fast. I presume the first migration is slow because memory fragmentation when an instance is running on the same compute node for a long time. I am looking for a solution considering the on my computing node I can have a little ram overcommit. Any case I am increasing the number of compute nodes to reduce it. Thanks Ignazio