Reduced SGA_TARGET, but SGA size not changing?

Question

I reduced the sga_taget from 1536M to 512M:
alter system set sga_target = 500M scope = memory;
System altered.
select VERSION from v$instance;
VERSION
10.2.0.3.0
show parameter sga
NAME TYPE VALUE
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 1536M
sga_target big integer 512M
But the real memory still showing the original value
show sga
Total System Global Area 1610612736 bytes
Fixed Size 2030456 bytes
Variable Size 1509950600 bytes
Database Buffers 83886080 bytes
Redo Buffers 14745600 bytes
why is that while it's a dynamic parameter?
Thanks a lot for any help.
Edited by: user10484253 on May 13, 2011 8:36 AM
Edited by: user10484253 on May 13, 2011 8:39 AM
Edited by: user10484253 on May 13, 2011 8:41 AM

Answer 1

I would suggest you to check v$sgastat to find out the exact SGA memory you are using currently instead of using "SHOW SGA" when when you set SGA_MAX_SIZE & SGA_TARGET initialization parameters.
Below is a sample output from one of my test dbs. As you can see below my SGA size is only 1GB.
SHOW SGA shows 2GB thats because I have set SGA_MAX_SIZE to 2gb ( which only means that I can grow my sga up till 2 gig , it may not be my current sga size).
you can try increasing or decreasing SGA_TARGET and check memory usage on OS level to see the difference.
SQL>show parameter sga
NAME                                 TYPE        VALUE
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     FALSE
sga_max_size                         big integer 2000M
sga_target                           big integer 1008M
SQL>show sga
Total System Global Area 2087780352 bytes
Fixed Size                  2155336 bytes
Variable Size            1744833720 bytes
Database Buffers          318767104 bytes
Redo Buffers               22024192 bytes
SQL>select name, round(sum(mb),1) mb
  2        from (
  3      select case when name = 'buffer_cache' then 'db_cache_size'
  4                  when name = 'log_buffer'   then 'log_buffer'
  5                  else pool
  6              end name,
  7              bytes/1024/1024 mb
  8                   from v$sgastat
  9           )group by name
10  /
NAME                  MB
db_cache_size        304
java pool            128
large pool            16
log_buffer            21
shared pool          528
                     2.1
6 rows selected.
SQL> -- V$SGA_DYNAMIC_FREE_MEMORY: Information about the amount of SGA memory available for future dynamic SGA resize operations.
SQL>select * from V$SGA_DYNAMIC_FREE_MEMORY;
CURRENT_SIZE
  1040187392- Krishna