Oracle MOS: LOW PGA HIT RATIO THOUGH OVER ALLOCATION COUNT IS NONE (Doc ID 284951.1) said: Over-allocating PGA memory can happen if the value of PGA_AGGREGATE_TARGET is too small to accommodate the PGA component un tunable (session context, cursor caches, etc) memory plus the minimum memory required to execute the work area workload. PGA is a memory region, that contains data and control information for a server process. The following example shows a query of this view. Use the V$PGA_TARGET_ADVICE view to determine the optimal value for the PGA_AGGREGATE_TARGET parameter and set its value to the optimal value, or to the maximum value possible, as described in "Using the V$PGA_TARGET_ADVICE View". Four sort operations have been executed, three were small (1 MB of input data) and one was bigger (100 MB of input data). Only 22 MB is required to sort 1 GB of data in one-pass size. For information about setting this parameter, see "Setting the Initial Value for PGA_AGGREGATE_TARGET". The V$WORKAREA_ACTIVE view displays the work areas that are active (or executing) in the database instance. Therefore, any query executed against the V$SQL_WORKAREA_HISTOGRAM view can be used on this view, with an additional predicate to select the desired value of the PGA_AGGREGATE_TARGET parameter. Statistics about the work area used by this sort operation are placed in the bucket defined by: HIGH_OPTIMAL_SIZE = 4194303 (4 MB minus 1 byte). Example 16-3 Querying the V$SQL_WORKAREA_HISTOGRAM View: Non-Empty Buckets. Each bucket is identified by a range of optimal memory requirements, specified by the values in the LOW_OPTIMAL_SIZE and HIGH_OPTIMAL_SIZE columns. Ideally, it should be small compared to total bytes processed. The Linux server needs a minimum of 3G swap and secure Linux set to permissive. Example 16-3 and Example 16-4 show two methods for querying this view. To accomplish this, Oracle Database first tries to maximize the number of optimal work areas for all memory-intensive SQL operations. This chapter describes how to tune the Program Global Area (PGA). In this particular example, the PGA_AGGREGATE_TARGET parameter should be set to at least 375 MB. 1.0 2,741,061.8 302,362.3 90.0 2, 1,721 The V$PGA_TARGET_ADVICE_HISTOGRAM view predicts how changing the value of the PGA_AGGREGATE_TARGET initialization parameter will affect the statistics in the V$SQL_WORKAREA_HISTOGRAM view. 3.0 2,741,061.8 272,980.1 91.0 0, 5,736 When automatic PGA memory management is disabled, the maximum size of a work area can be sized with the associated _AREA_SIZE parameter, such as the SORT_AREA_SIZE initialization parameter. Excessive PGA usage can lead to high rates of swapping. To set the initial value for PGA_AGGREGATE_TARGET: Determine how much of the total physical memory to reserve for the operating system and other non-Oracle applications running on the same system. The less Disk usage figure in this column, usually the better. "There is much worth noticing that often escapes the eye." Oracle Database reads and writes information in the PGA on behalf of the server process. The Program Global Area (PGA) is a private memory region that contains the data and control information for a server process. Table 16-1 Statistics in the V$PGASTAT View. You can join the V$SQL_WORKAREA view with the V$SQL view to relate a work area to a cursor, and with the V$SQL_PLAN view to precisely determine which operator in the plan uses a work area. Generally, adding PGA memory requires reducing memory for some SGA components—like the shared pool or buffer cache—because the overall memory dedicated to the database instance is often bound by the amount of physical memory available on the system. This post describes the installation of Oracle Database 19c 64-bit on Oracle Linux 7.6 x86-64 bit. Upgrade the database time zone file using the DBMS_DST package in 19c, ORA-56920: a prepare or upgrade window or an on-demand or datapump-job loading of a secondary time zone data file is in an active state, PRCD-1229 : An attempt to access configuration of database orcl was rejected because its version 12.2.0.1.0 differs from the program version 19.0.0.0.0, ORA-39701: database must be mounted EXCLUSIVE for UPGRADE or DOWNGRADE. Setting PGA to 24576MB can eliminate over allocation count and gain maximum hit ratio. Run a representative workload on the database instance and monitor its performance, as described in "Monitoring Automatic PGA Memory Management". In Figure 16-1, this happens when PGA_AGGREGATE_TARGET reaches 3 GB. If the work area is less than 40 MB, then the sort operation must perform several passes over the input data. Over-allocating PGA memory can happen if the PGA_AGGREGATE_TARGET value is too small to accommodate the PGA other component and the minimum memory required to execute the work area workload. An example of this type of Linux installation can be seen, Automating RMAN backup Jobs with CRON utility Change ), You are commenting using your Twitter account. However, PGA memory usage may exceed the PGA_AGGREGATE_TARGET setting at times due to the following reasons: The PGA_AGGREGATE_TARGET setting acts as a target, and not a limit. The V$SQL_WORKAREA_HISTOGRAM view shows the number of work areas executed with optimal, one-pass, and multi-pass memory size since instance startup. -- ----------------------------- --------------------. Use the V$PGA_TARGET_ADVICE view to ensure that the PGA_AGGREGATE_TARGET value is not set within the over-allocation zone, as described in "Using the V$PGA_TARGET_ADVICE View". Running one-pass executions does not require a large amount of memory. MULTIPASSES_EXECUTIONS-number of times, this work area used temporary tablespace in multiple times to get it finished. If you cannot reduce memory from the SGA components, consider adding more physical memory to the system. Example 16-1 Calculating Cache Hit Percentage. This value can then be tuned and dynamically modified at the instance level.
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