Any segments whose blocks tend to be accessed with less frequency should be assigned to the recycle pool so that it does not flush the other segments, either in the default cache or the keep pool.
Segments that contain frequently accessed blocks should be
assigned to the keep buffer pool so that the blocks of those segments will
not be inadvertently removed, thus impacting performance.
the keep buffer pool is used to
retain objects in memory that are likely to be reused. Keeping these
objects in memory reduces physical I/O operations. For optimal performance
you should make sure that you tune your SQL to reduce logical I/O
operations as well as physical operations.