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RAM Allocation at Oracle Instance Startup Tips

Donald K. Burleson

RAM Allocation at Oracle Instance Startup

To fully understand RAM usage, it is interesting to observe Oracle RAM and CPU allocation at startup time.  

Figure 1.1 is a time-based snapshot of an Oracle database’s CPU and RAM resource consumption at database startup time.  We see the RAM allocated when the SGA is started and the RAM usage remains relatively constant after this point.  As for CPU usage, we see that the CPU stress peaks during database mounting and declines as the background processes become idle.

It is important to know that the allocation of RAM memory for an Oracle server can be done solely with mathematics, and no expensive performance monitors are required to properly estimate the initial RAM demands of your Oracle database.  Once your Oracle database is configured, you can change the RAM according to demand.

Prior to release 8.1.7, the most difficult part of Oracle RAM optimization in any environment was accurately predicting the high water mark of dedicated, connected user sessions once the instance was started.  This was because of a bug in the v$resource_limit view. After release 8.1.7, you can use v$resource_limit to see the high water mark of connected sessions since startup time. 

If we have an unexpected spike of dedicated, connected sessions, it is possible that we would exceed the amount of RAM on the server, causing active programs’ RAM regions to go out to the swap disk.  In sum, the goal is to fully allocate RAM without ever experiencing RAM paging and to re-allocate RAM within the Oracle SGA to optimize performance.

To see the size of your SGA, you can issue the show sga  command from SQL*Plus, as shown below.  The output of the show sga command appears here:

SQL> connect system/manager as sysdba;
SQL> show sga

Total System Global Area                        405323864 bytes
Fixed Size                                          49240 bytes
Variable Size                                   354066432 bytes
Database Buffers                                 49152000 bytes
Redo Buffers                                      2056192 bytes


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