I blogged about Dynamic Resource Mastering (DRM) in RAC here . DRM freezes the global resources during the reconfiguration event and no new resources can be allocated during the reconfiguration. This freeze has a dramatic effect of inducing huge amount of waits for gc buffer busy [acquire|release] events and other gcs drm freeze release, gcs remaster events. In database version 12c, DRM has been improved further.
A major improvement I see is that not all resources are frozen at any time. Essentially, resources are broken down in to partitions and only a resource partition is frozen. This improvement should decrease the impact of DRM related waits tremendously.
LMON Trace file
Following shows the snippet from the LMON trace file. As you see, only one resource partition is frozen, at-a-time. Resources in the first partition is frozen, completes the resource remastering tasks, and unfreezes that resource partition. Then freezes next resource partition and continues until all resources are remastered.