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Oracle 11g memory_target parameter

Oracle Tips by Burleson Consulting


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.

 

Oracle will automatically set sga_max_size to 60% of the setting for memory_target

 

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.

Note:  When using AMM (by setting memory_target, and/or sga_target, the values for the ?traditional? pool parameters (db_cache_size, shared_pool_size, &c) are not ignored.  Rather, they will specify the minimum size that Oracle will always maintain for each sub-area in the SGA.

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 Oracle 11g documentation notes that the memory_target parameter specifies the system-wide RAM and explains that the memory_max_target=memory_target if you do not explicitly set memory_max_target.

The docs also note that the memory_max_target specifies the maximum amount of RAM that you would allocate for all of Oracle, all SGA and PGA regions combined.

 

Issues with memory_target in 11g - Steve Karam

I was working on an 11g client yesterday.  Their settings were:

memory_target = 16G
sga_target = 0
db_cache_size = 0
shared_pool_size = 0
etc.

For whatever reason, Oracle would not allocate over 64 MB for the buffer cache.  We tried to set the minimum by doing:

alter system set db_cache_size = 6G

But we received this error:

ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache

Pretty weird, right? 

So we set memory_target = 0, which set the sga_target = 640MB and kept all the other pools at 0.  Then we set the sga_target to 0, and were finally allowed to use more memory for the buffer cache.

So even though the new AMM parameter was set very high, Oracle was still going by the rules of the underlying sga_target parameter, which was incorrect.

I wanted to pass this around so you all know that memory_target in 11g is buggy.  For that matter, sga_target has always seemed buggy as well with the way it can harm system performance.

 

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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