Modifying Memory Settings - Manual Shared Memory Management

Before you modify memory settings for manual shared memory management, use the Buffer Cache Advisor graph in Oracle Enterprise Manager Database Express (EM Express) to predict the estimated percentage of reads saved by using a different database cache size. This section assumes that automatic memory management is disabled, and that manual shared memory management is enabled.

To predict the percentage of reads saved for different database cache sizes:

1. In EM Express, from the Configuration menu, select Memory.

   The Memory Management page appears. In the Memory Settings section, the Management Mode value is Manual in the SGA Memory subsection.This indicates that Manual Shared Memory Management is enabled for the database.

2. Use the Buffer Cache Advisor graph (which appears to the right of the Memory Settings section) to predict the percentage of reads saved for potential database cache sizes.

   
   Description of the illustration buffer_cache_advisor_graph.gif

   In the Buffer Cache Advisor graph:

   • Potential values for the DB_CACHE_SIZE initialization parameter (in MB) are represented on the horizontal axis of the graph. The current setting of the DB_CACHE_SIZE initialization parameter is indicated by a blue dot.

   • The corresponding values of reads saved are represented on the vertical axis of the graph. The plotted values are expressed as a percentage relative to the current setting of the DB_CACHE_SIZE initialization parameter.

     Negative values represent the percentage of an increase in reads (when the memory allotted to Oracle is smaller than the current setting), while positive values represent the percentage of decrease in reads (when the memory alloted to Oracle is larger than the current setting.

   An orange line on the graph plots different values that can be specified for the DB_CACHE_SIZE initialization parameter. Click any dot on the orange line to see a prediction of the percentage of reads saved for the DB_CACHE_SIZE value represented by that dot.

   In this figure, the Buffer Cache Advisor graph indicates that increasing the current value of the DB_CACHE_SIZE initialization parameter will not increase the percentage of reads saved.

3. To change the value of the DB_CACHE_SIZE initialization parameter:

   1. Click Configure Memory on the Memory Management page.

      The Initialization Parameter page appears.

   2. Select the DB_CACHE_SIZE initialization parameter and click Set.

      The Set Initialization Parameter page appears.

   3. In the Scope field, enter the scope for this change.

      See "Viewing and Modifying Initialization Parameters" for more information about specifying a scope of MEMORY, or SPFile, or both.

   4. In the Value field, enter the new value for the DB_CACHE_SIZE initialization parameter.

   5. Click OK.

      A confirmation message appears.