This short article examines the new PGA memory management in Oracle 11g,
using the new memory_target and memory_max_target
parameters.
Prior to Oracle 11g, the DBA set the sga_target and
sga_max_size parameters, allowing Oracle to reallocate RAM
within the SGA. The PGA was independent, as governed by
the pga_aggregate_target parameter.
Now in Oracle 11g we see the memory_max_target parameter
which governs the total maximum RAM for both the PGA and SGA
regions and the new memory_target parameter which governs
the existing sizes. This allows RAM to be de-allocated
from the SGA and transferred to the PGA.
This is an important Oracle11g new feature because it lays the
foundation for inter-instance RAM memory sharing.
We are seeing the
second age of mainframe computing, and server consolidation
where it's not uncommon to find a dozen instances on a single
large server.
As of Oracle 10g, Automatic Memory Management (AMM) only allows
shifting of RAM within the confines of sga_max_size.
Now in Oracle 11g, we see this new Automatic Shared Memory
Management (ASMM). The memory_target parameter is dynamic
(changeable with "alter system" commands), whereby RAM
can be de-allocated from an instances SGA/PGA and re-allocated
to another instance.
The blog claims that the Oracle 11g documentation notes:
MEMORY_TARGET specifies the Oracle system-wide usable memory. The database
tunes memory to the the MEMORY_TARGET value, reducing or enlarging the SGA
and PGA as needed. In a text initialization parameter file, if you omit the
line for MEMORY_MAX_TARGET and include a value for MEMORY_TARGET, the
database automatically sets MEMORY_MAX_TARGET to the value of MEMORY_TARGET.
If you omit the line for MEMORY_TARGET and include a value for
MEMORY_MAX_TARGET, the MEMORY_TARGET parameter defaults to zero.
After startup, you can then dynamically change MEMORY_TARGET to a non-zero
value, provided that it does not exceed the value of MEMORY_MAX_TARGET.
