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.

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