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Using the Oracle v$pgastat view

The v$pgastat view provides instance level summary statistics on the PGA usage and the automatic memory manager. The following script provides excellent overall usage statistics for all Oracle connections.

check_pga.sql
column name format a30
column value format 999,999,999

select
name,
value
from
v$pgastat
;
The output of this query might look like the following:
NAME VALUE
------------------------------------------------------ ----------
aggregate PGA auto target 736,052,224
global memory bound 21,200
total expected memory 141,144
total PGA inuse 22,234,736
total PGA allocated 55,327,872
maximum PGA allocated 23,970,624
total PGA used for auto workareas 262,144
maximum PGA used for auto workareas 7,333,032
total PGA used for manual workareas 0
maximum PGA used for manual workareas 0
estimated PGA memory for optimal 141,395
maximum PGA memory for optimal 500,123,520
estimated PGA memory for one-pass 534,144
maximum PGA memory for one-pass 52,123,520


In the above display from v$pgastat we see the following statistics.

* Aggregate PGA auto target – This column gives the total amount of available memory for Oracle connections. As we have already noted, this value is derived from the value on the init.ora parameter pga_aggregate_target.
* Global memory bound – This statistic measures the max size of a work area, and Oracle recommends that whenever this statistics drops below one megabyte, then you should increase the value of the pga_aggregate_target parameter.
* Total PGA allocated – This statistic display the high-water mark of all PGA memory usage on the database. You should see this value approach the value of pga_aggregate_target as usage increases.
* Total PGA used for auto workareas – This statistic monitors RAM consumption or all connections that are running in automatic memory mode. Remember, not all internal processes may use the automatic memory feature. For example, Java and PL/SQL will allocate RAM memory, and this will not be counted in this statistic. Hence, we can subtract value to the total PGA allocated to see the amount of memory used by connections and the RAM memory consumed by Java and PL/SQL.
* Estimated PGA memory for optimal/one-pass – This statistic estimates how much memory is required to execute all task connections RAM demands in optimal mode. Remember, when Oracle experienced a memory shortage, he will invoke the multi-pass operation. This statistics is critical for monitoring RAM consumption in Oracle, and most Oracle DBA’s will increase pga_aggregate_target to this value.


The above is an excerpt from the "Oracle UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.

 

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