If the Cache Hit Ratio is less then 90%then Increase the DB_BLOCK_BUFFERS.
This parameter is the Number of theDB_BLOCKS.
So if theDB_BLOCK_SIZE = 8192 and DB_BLOCK_BUFFERS = 50000 then, Since the DB_BLOCK_BUFFERS
is in Units of the DB_BLOCK_SIZE
these 2  effects the actual size(in Number of Bytes) allocated . In Our case the
actual size will be 8192*50000 = 406,600,000 (400 MB Only).
Depending on the Resources Available u can grant upto 40% of the Physical
Memory available to this Parameter.
In Oracle9i Onward its Possible to have multiple DB_blockSize within a Single Database. This is a Powerful Feature
Particularly When U are Running a Hybrid
System( DSS/OLTP Mixed in a Single Database Instance).
 

Memory Tuning Aspects

First try to get the size of the Sharable memoryFrom V$sqlareaWhere the Executions are very Frequent. Let’s assume that any Code that is executed more than five times can be considered as frequently used.
      Select Sum (Sharable_mem) From
      V$sqlarea where executions > 5
Again this 5 is just for example, based on your application One has to judge by when something can be classified as frequently used.

Have a Oracle Question
Do you have an Oracle Question?

Oracle Books
Oracle Certification, Database Administration, SQL, Application, Programming Reference Books

Oracle Application
Oracle Application Hints and Tips

Oracle Home
Oracle Database, SQL, Application, Programming Tips