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How much memory is truly used by my Oracle instance?

There are many posts about the amount of memory that is taken by the Oracle database executables and the database SGA and PGA. The reason for adding yet another one on this topic is a question I recently gotten, and the complexities which surrounds memory usage on modern systems. The intention for this blogpost is to show a tiny bit about page sharing of linux for private pages, then move on to shared pages, and discuss how page allocation looks like with Oracle ASMM (sga_target or manual memory).

The version of linux in this blogpost is Oracle Linux 7.2, using kernel: 4.1.12-37.6.3.el7uek.x86_64 (UEK4)
The version of the Oracle database software is (july 2016).

Automatic shared memory resize with disabled ASMM/AMM

Thanks to Nigel Antell – he has posted a comment on one of Tanel Poder’s blog notes with a link to MOS Doc ID 1269139.1. In this note a parameter called _MEMORY_IMM_MODE_WITHOUT_AUTOSGA is described. It allows Oracle 11gR2 to adjust sizes of shared memory structures even if Automatic (Shared) Memory Management have been explicitly disabled; with the main purpose of avoiding ORA-04031 error. This is nice.
PS. I wonder if it was done by Oracle development specifically due to many customers don’t like ASMM (for ex. because of negative impact on application) but encounter ORA-04031 from time to time, which results in many opened SRs. Who knows.