Oracle granule

SGA memory is allocated in unit of contiguous memory chunks called granule. The SGA memory components(shared pool,buffer cache, redo log buffer,java pool,streams pool,large pool) are sized as multiples of granules.

The granule size is determined based on the amount of memory requested at the instance startup. It is based on the SGA_MAX_SIZE. If MEMORY_MAX_TARGET is specified, then SGA_MAX_SIZE defaults to MEMORY_MAX_TARGET for the purpose of sizing the granule. Once set, the granule size does not change for the life of the instance.
However granule maximum size = 16MB on 32-bit platforms.

SQL> show parameter memory_max_target

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
memory_max_target big integer 800M

SQL> select bytes/1024/1024 bytes_Mb from v$sgainfo where name like 'Granule Size';

BYTES_MB
----------
4

RDBMS	SGA_MAX_SIZE (or memory_max_target)	GRANULE SIZE
9.2	<= 128MB	4MB
	> 128MB	16MB
10.2	<= 1GB	4MB
	> 1GB	16MB
11gR1	<= 1GB	4MB
	1Gb - 4GB	16MB
	4Gb - 16GB	64MB
	16Gb - 64GB	256MB
	> 64GB	512MB
11gR2 (and 11gR1 with patch 8813366 applied *)	< 1Gb	4Mb
	1Gb - 8Gb	16Mb
	8Gb - 16Gb	32Mb
	16Gb - 32Gb	64Mb
	32Gb - 64Gb	128Mb
	64Gb - 128Gb	256Mb
	> 128Gb	512Mb

How to test from above table. If I need granule size 16Mb, I have to Memory 1Gb - 4GB for SGA???

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
PL/SQL Release 11.1.0.6.0 - Production
CORE 11.1.0.6.0 Production
TNS for Linux: Version 11.1.0.6.0 - Production
NLSRTL Version 11.1.0.6.0 - Production

SQL> !free -m
total used free shared buffers cached
Mem: 888 849 38 0 2 740

I have only 900Mb for Physical Memory. Find Idea to test then created /tmp/pfile file for modified MEMORY_MAX_TARGET parameter.

SQL> create pfile='/tmp/pfile' from spfile;

File created.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

then faked /dev/shm size (this's just test for granule size to fake memory_max_target parameter)

Filesystem Size Used Avail Use% Mounted on
tmpfs 4.0G 0 4.0G 0% /dev/shm

then modified /tmp/pfile (MEMORY_MAX_TARGET=4G) - If MEMORY_MAX_TARGET is specified, then SGA_MAX_SIZE defaults to MEMORY_MAX_TARGET.

SQL> startup pfile='/tmp/pfile';
ORACLE instance started.

Total System Global Area 2058981376 bytes
Fixed Size 1300968 bytes
Variable Size 1644168728 bytes
Database Buffers 402653184 bytes
Redo Buffers 10858496 bytes
Database mounted.
Database opened.

SQL> show parameter memory_max_target

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
memory_max_target big integer 4G

SQL> select bytes/1024/1024 bytes_Mb from v$sgainfo where name like 'Granule Size';

BYTES_MB
----------
16

Used granule size 16M.
If we set a value in the spfile that is not a multiple of the granule size, the actual size allocated will be rounded up to the nearest granule.

SQL> select bytes/1024/1024 bytes_Mb from v$sgainfo where name like 'Granule Size';

BYTES_MB
----------
4

SQL> alter system set java_pool_size=4M;

System altered.

SQL> show parameter java_pool_size

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
java_pool_size big integer 4M

SQL> alter system set MEMORY_MAX_TARGET=4G scope=spfile;

System altered.

SQL> shutdown

SQL> startup

SQL> select bytes/1024/1024 bytes_Mb from v$sgainfo where name like 'Granule Size';

BYTES_MB
----------
16

SQL> show parameter java_pool_size

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
java_pool_size big integer 16M

The java_pool_size 4M in the spfile, after increased granule size, then the actual allocation for the java_pool_size will be rounded up to 16M.