Components and Granules in the SGA

The SGA comprises a number of memory components, which are pools of memory used to satisfy a particular class of memory allocation requests.
Examples of memory components include

* the shared pool (used to allocate memory for SQL and PL/SQL execution)
* the java pool (used for java objects and other java execution memory)
* the buffer cache (used for caching disk blocks)

All SGA components allocate and deallocate space in units of granules.
Oracle Database tracks SGA memory use in internal numbers of granules for each SGA component.

The memory for dynamic components in the SGA is allocated in the unit of granules. Granule size is determined by total SGA size.
Generally speaking, on most platforms, if the total SGA size is equal to or less than 1 GB, then granule size is 4 MB.
For SGAs larger than 1 GB, granule size is 16 MB.

You can query the V$SGAINFO view to see the granule size that is being used by an instance.
The same granule size is used for alldynamic components in the SGA.

Source: Oracle Documentation