Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 4 ■ MEMORY STRUCTURES
In Oracle9i, the various SGA components must be manually sized by the DBA but, starting
in Oracle 10g, there is a new option to consider: automatic SGA memory management,
whereby the database instance will allocate and reallocate the various SGA components at
runtime, in response to workload conditions. When using the automatic memory management
with Oracle 10g, it is a matter of simply setting the SGA_TARGET parameter to the desired
SGA size, leaving out the other SGA-related parameters altogether. The database instance will
take it from there, allocating memory to the various pools as needed and even taking memory
away from one pool to give to another over time.
Regardless of whether you are using automatic or manual memory management, you will
find that memory is allocated to the various pools in units called granules. A single granule is
an area of memory either 4MB, 8MB, or 16MB in size. The granule is the smallest unit of allocation,
so if you ask for a Java pool of 5MB and your granule size is 4MB, Oracle will actually
allocate 8MB to the Java pool (8 being the smallest number greater than or equal to 5 that is a
multiple of the granule size of 4). The size of a granule is determined by the size of your SGA
(this sounds recursive to a degree, as the size of the SGA is dependent on the granule size). You
can view the granule sizes used for each pool by querying V$SGA_DYNAMIC_COMPONENTS. In fact,
we can use this view to see how the total SGA size might affect the size of the granules:
sys@ORA10G> show parameter sga_target
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
sga_target
big integer 576M
sys@ORA10G> select component, granule_size from v$sga_dynamic_components;
COMPONENT
GRANULE_SIZE
------------------------- ------------
shared pool 4194304
large pool 4194304
java pool 4194304
streams pool 4194304
DEFAULT buffer cache 4194304
KEEP buffer cache 4194304
RECYCLE buffer cache 4194304
DEFAULT 2K buffer cache 4194304
DEFAULT 4K buffer cache 4194304
DEFAULT 8K buffer cache 4194304
DEFAULT 16K buffer cache 4194304
DEFAULT 32K buffer cache 4194304
OSM Buffer Cache 4194304
13 rows selected.
In this example, I used automatic SGA memory management and controlled the size of
the SGA via the single parameter SGA_TARGET. When my SGA size is under about 1GB, the granule
is 4MB. When the SGA size is increased to some threshold over 1GB (it will vary slightly
from operating system to operating system and even from release to release), I see an