This post is from one of the emails I have recieved from my friend Mackenzie D’Cruz, he experienced this issue on his production environment.
This information is published by Don Burleson at http://www.dba-oracle.com. So this is just a copy to let our readers know about this issue.

Beware, following new upgrades to 11gR2, you can find that AMM has reappeared, even if you think that you have disabled it.

Oracle Automatic Shared Memory Management (ASMM) was introduced in Oracle 10g for small databases where a one-sized-fits-all approach was used in small hands-off databases that did not have a DBA to intelligently allocate the SGA pools.

When a small system has a too small RAM region for the SGA, Oracle AMM will cause a performance problem with RAM thrashing and it tries to readjust all of the RAM regions.

In all large mission-critical databases, the DBA will disable AMM because an intelligent human will always do a better job than a simple software solution, especially when you need to intelligently reconfigure the SGA in anticipation of a changing workload.

The following references will be helpful in researching the behavior of AMM:

Disabling AMM in Oracle 11g Release 2

You can enable and disable AMM by changing the value of these parameters to non-zero settings:

  • sga_target
  • sga_max_size
  • pga_aggregate_target
  • sga_max_size
  • db_cache_size
  • shared_pool_size
  • java_pool_size

nfortunately a new feature in 11gR2 turns AMM on unless a hidden parameter called _memory_imm_mode_without_autosga is set to FALSE.

Important 11gR2 Note:  Even if AMM has been disabled in Oracle 10g, AMM may reappear in Oracle 11g unless the following parameter is set:

alter system set “_memory_imm_mode_without_autosga”=FALSE scope=both;

There is a MOSC note about AMM in 11gR2 that notes that this regression back to AMM is the expected behavior in 11.2 for immediate memory allocation requests and that Oracle added this behavior as a 11gR2 new feature when automatic memory management was disabled.

The 11.2 Oracle support metalink  note on AMM regression is note number 1269139.1, titled:

SGA Re-Sizes Occurring Despite AMM/ASMM Being Disabled(MEMORY_TARGET/SGA_TARGET=0)

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