ISM and DISM Feature on Solaris for Oracle Server Tuning

July 2, 2011 - PRLog -- Oracle System Global Area populates as Shared Memory segment in Unix. Thus, we need to define kernel parameters of shared memory segment like SHMMAX, SHMMIN etc for appropriate SGA of Oracle instance. System Global Area is memory region of Oracle database. For performance enhancement and if your system has sufficient physical memory then sizing of SGA would set appropriate.

Solaris operating systems, Oracle instance utilizes ISM feature. ISM called as Intimate Shared Memory in Solaris. ISM was introduced in Solaris for performance enhancement of application and server tuning. If Oracle instance creates SGA in ISM then System Global Area automatically locked in shared memory segment. Therefore, performance is boosting up the Oracle SGA and database.

New feature of Oracle was introduced to resizing SGA on demand and without bouncing database. If Oracle SGA is using ISM and it locked in shared memory segment then there is no scope to resize such memory pages as well as expanding memory parameter without bouncing database. Hence, there is another feature is also available called Dynamic Intimate Shared Memory in Solaris. This DISM feature enables memory resizing of SGA.

ISM and DISM both features are available in Solaris 10 for server tuning and performance enhancement of Oracle server. Oracle memory new feature called SGA_TARGET and MEMORY_TARGET uses DISM. Oracle Database can abandon memory from one dynamic SGA element and allocate it to another element. Using DISM, Oracle can dynamically allocating and de-allocating memory to every part of SGA like Buffer cache, Shared Pool, Large Pool etc.

For ISM and DISM memory allocation to Oracle SGA there are certain limitations. If Oracle SGA uses ISM then Solaris system allocates implicit lock on shared memory and if Oracle SGA uses DISM then Solaris system doesn’t allocate implicit locking shared memory. Reason behind this scenario is dynamic resizing of memory pages in Unix pages. Therefore, Solaris server uses explicit locking whenever DISM is used by Oracle SGA. This explicit locking mechanism facilitates dynamic memory pages expanding and decreasing feature. ISM and DISM both features support VLM very Large Memory pages on 64 bit Solaris systems.

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