Automatic PGA Memory Management

                                        Automatic PGA Memory Management

                                        -------------------------------------------------

Process Global Area,  often known as the Program Global Area (PGA) resides in the process private memory of the server process. It contains global variables and data structures and control information for a server process. example of such information is the runtime area of a cursor. Each time a cursor is executed, a new runtime area is created for that cursor in the PGA memory region of the server process executing that cursor.

The performance of complex long running queries, typical in a DSS environment ,depend to a large extent on the memory available in the Program Global Area (PGA).which is also called work area. The size of a work area can be controlled and tuned. Generally, bigger work areas can significantly improve the performance of a particular operator at the cost of higher memory consumption . Ideally, the size of a work area is big enough that it can accommodate the input data and auxiliary memory structures allocated by its associated SQL operator. This is known as the optimal size of a work area (e.g.a memory sort). When the size of the work area is smaller than optimal (e.g. a disk sort), the response time increases, because an extra pass is performed over part of the input data. This is known as the one-pass size of the work area. Under the one-pass threshold, when the size of a work area is far too small compared to the input data size, multiple passes over the input data are needed. This could dramatically increase the response time of the operator. This is known as the multi-pass size of the work area.

In Oracle8i administrators sized the PGA by carefully adjusting a number of

initialization parameters, such as, SORT_AREA_SIZE, HASH_AREA_SIZE, BITMAP_ MERGE_AREA_SIZE, and CREATE_BITMAP_AREA_SIZE, etc.

Starting with Oracle9i, an option is provided to completely automate the management of PGA memory.  Administrators merely need to specify the maximum amount of PGA memory available to an instance using a newly introduced initialization parameter PGA_AGGREGATE_TARGET.

The database server automatically distributes this memory among various active queries in an intelligent manner so as to ensure maximum performance benefits and the most efficient utilization of memory. Furthermore, Oracle9i and newer releases can adapt itself to changing workload thus utilizing resources efficiently regardless of the load on the system. The amount of the PGA memory available to an instance can be changed dynamically by altering the value of the PGA_AGGREGATE_TARGET parameter making it possible to add to and remove PGA memory from an active instance online. Since the database engine itself is better equipped to determine SQL execution memory requirements, database administrators should use this feature and not try to tune the PGA manually. This should translate to better throughput for large number of users on the system as well as improved response time for queries.

The automatic SQL execution memory management feature is enabled by setting the parameter WORKAREA_SIZE_POLICY to AUTO and by specifying a size of PGA_AGGREGATE _TARGET in the initialization file. These two parameters can also be set dynamically using the ALTER SYSTEM command. In the absence of either of these parameters, the database will revert to manual PGA management mode. In Oracle9i Release 2, an advisory for PGA_AGGREGATE_TARGET was introduced. Just like in Buffer Cache Advisory, the PGA Advisory will suggest the appropriate size for PGA memory and thus make PGA tuning an even simpler task.

Version specific notes:

Until 9iR2, PGA_AGGREGATE_TARGET parameter controls the sizing of workareas for all dedicated server connections, but it has no effect on shared servers (aka MTS) connections and the *_AREA_SIZE parameters will take precedence in this case.

In 10g, PGA_AGGREGATE_TARGET controls workareas allocated by both dedicated and shared connections.

As of 11g, Automatic Memory Management (AMM) expands to managing both SGA and PGA memory.   Under memory pressure for PGA memory, SGA memory will be re-allocated for use by a process to accommodate workarea needs.  On the flip-side, if PGA memory is under allocated, memory can be added to the auto-tuned components in the SGA beyond the original SGA configuration.

NOTE:   With AMM, setting an explicit value for PGA_AGGREGATE_TARGET will act as a minimum setting that AMM will not shrink below.

  See Note:443746.1 for more information.

How To Tune PGA_AGGREGATE_TARGET

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The first question we will have when we set this parameter is what is the best value for it?  

To determine the appropriate setting for PGA_AGGREGATE_TARGET parameter we recommend to follow the following steps

1- Make a first estimate for PGA_AGGREGATE_TARGET based on the following rule

- For OLTP systems

   PGA_AGGREGATE_TARGET  = (<Total Physical Memory > * 80%) * 20%

- For DSS systems

   PGA_AGGREGATE_TARGET = (<Total Physical Memory > * 80%) * 50%

So for example, if we have an Oracle instance configured on system with 16G of Physical memory, then the suggested PGA_AGGREGATE_TARGET parameter value we should start with incase we have OLTP system is (16 G * 80%)*20% ~= 2.5G and incase we have DSS system is (16 G * 80%)* 50% ~= 6.5 G.

In the above equation, we assume that 20% of the memory will be used by the OS, and in OLTP system 20% of the remaining memory will be used for PGA_AGGREGATE_TARGET  and the remaining memory is going for Oracle SGA memory and non-oracle processes memory. So make sure that you have  enough memory for your SGA and also for non-oracle processes

2- A second step in tuning the PGA_AGGREGATE_TARGET is to monitor performance using available PGA statistics and see if PGA_AGGREGATE_TARGET is under sized or over sized. Several dynamic performance views are available for this purpose:

- V$PGASTAT

This view provides instance-level statistics on the PGA memory usage and the automatic PGA memory manager. For example:

SELECT * FROM V$PGASTAT;

NAME                                                                     VALUE

----------------------------------------                    ----------------

Aggregate PGA target parameter                              524288000 bytes

aggregate PGA auto target                                       463435776 bytes

global memory bound                                             25600 bytes

total PGA inuse                                                       9353216 bytes

total PGA allocated                                                 73516032 bytes

maximum PGA allocated                                        698371072 bytes

total PGA used for auto workareas                                      0 bytes

maximum PGA used for auto workareas                                560744448 bytes

total PGA used for manual workareas                  0 bytes

maximum PGA used for manual workareas              0 bytes

over allocation count                                                             0 bytes

total bytes processed                                              4.0072E+10 bytes

total extra bytes read/written                                                  3.1517E+10 bytes

cache hit percentage                                                  55.97 percent

Main statistics to look at

(a) AGGREGATE PGA AUTO TARGET : This gives the amount of PGA memory Oracle can use for work areas running in automatic mode. This part of memory represent the tunable part of PGA memory,i.e. memory allocated for intensive memory SQL operators like sorts, hash-join, group-by, bitmap merge and bitmap index create. This memory part can be shrinked/expanded in function of the system load. Other parts of PGA memory are known as untunable, i.e. they require a size that can't be negociated  (e.g. context information for each session, for each open/active cursor, PL/SQL or Java memory).

So, the aggregate PGA auto target should not be small compared to the value of  PGA_AGGREGATE_ TARGET. You must ensure that enough PGA memory is left for work areas  running in automatic mode.

(b) TOTAL PGA USED FOR AUTO WORKAREA: This gives the actual tunable PGA memory used by the system. The 'maximum PGA used for auto workareas' gives the maximum reached by previous statistic since instance startup.

(c) TOTAL PGA IN USED: This gives the total PGA memory in use. The detail of this value can be found in the PGA_USED_MEM column of the v$process view.

 (d) OVER ALLOCATION COUNT: Over-allocating PGA memory can happen if the value of PGA_AGGREGATE _ TARGET is too small to accommodate the untunable PGA memory part plus the minimum memory required to execute the work area workload. When this happens,  Oracle cannot honor the initialization parameter PGA_AGGREGATE_TARGET, and extra PGA memory needs to be allocated. over allocation count is the number of time the system was detected in this state since database startup. This count should ideally be equal to zero.

 (e) CACHE HIT PERCENTAGE: This metric is computed by Oracle to reflect the performance of the PGA memory component. It is cumulative from instance start-up. A value of 100% means that all work areas executed by the system since instance start-up have used an optimal amount of PGA memory. This is, of course, ideal but rarely happens except maybe for pure OLTP systems. In reality, some work areas run one-pass or even multi-pass,  depending on the overall size of the PGA memory. When a work area cannot run optimally, one or more extra passes is performed over the input data. This reduces the cache hit percentage in proportion to the size of the input data and the number of extra passes performed. this value if computed from the "total bytes processed" and "total extra bytes read/written" statistics available in the same view using the following formula:

                                total bytes processed * 100

PGA Cache Hit Ratio =  ------------------------------------------------------

                       (total bytes processed + total extra bytes read/written)

- V$SQL_WORKAREA_HISTOGRAM

This view shows the number of work areas executed with optimal memory size, one- pass memory size, and multi-pass memory size since instance start-up. Statistics in this view are subdivided into buckets that are defined by the optimal memory requirement of the work area. Each bucket is identified by a range of optimal  memory requirements specified by the values of the columns LOW_OPTIMAL_SIZE and HIGH_OPTIMAL_SIZE.

Example :

The following query shows statistics for all nonempty buckets.

SELECT LOW_OPTIMAL_SIZE/1024 LOW_KB,(HIGH_OPTIMAL_SIZE+1)/1024 HIGH_KB,

 OPTIMAL_EXECUTIONS, ONEPASS_EXECUTIONS, MULTIPASSES_EXECUTIONS  FROM   V$SQL_WORKAREA_HISTOGRAM WHERE  TOTAL_EXECUTIONS != 0;

The result of the query might look like the following:

LOW_KB HIGH_KB OPTIMAL_EXECUTIONS ONEPASS_EXECUTIONS MULTIPASSES_EXECUTIONS

------ ------- ------------------ ------------------ ----------------------

8      16      156255             0                   0

16     32      150                0                   0

32     64      89                 0                   0

64     128     13                 0                   0

128    256     60                 0                   0

256    512     8                  0                   0

512    1024    657                0                   0

1024   2048    551                16                  0

2048   4096    538                26                  0

4096   8192    243                28                  0

8192   16384   137                35                  0

16384  32768   45                 107                 0

32768  65536   0                  153                 0

65536  131072  0                  73                  0

131072 262144  0                  44                  0

262144 524288  0                  22                  0

The query result shows that, in the 1024 KB to 2048 KB bucket, 551 work areas used an optimal amount of memory, while 16 ran in one-pass mode and none ran in multi-pass mode. It also shows that all work areas under 1 MB were able to run in optimal mode.

You can also use V$SQL_WORKAREA_HISTOGRAM to find the percentage of times work  areas were executed in optimal, one-pass, or multi-pass mode since start-up.

Example :

SELECT OPTIMAL_COUNT, ROUND(OPTIMAL_COUNT*100/TOTAL, 2) OPTIMAL_PERC,     ONEPASS_COUNT, ROUND(ONEPASS_COUNT*100/TOTAL, 2) ONEPASS_PERC,      MULTIPASS_COUNT, ROUND(MULTIPASS_COUNT*100/TOTAL, 2) MULTIPASS_PERC

FROM       (SELECT DECODE(SUM(TOTAL_EXECUTIONS), 0, 1, SUM(TOTAL_EXECUTIONS)) TOTAL,

               SUM(OPTIMAL_EXECUTIONS) OPTIMAL_COUNT,

               SUM(ONEPASS_EXECUTIONS) ONEPASS_COUNT,

               SUM(MULTIPASSES_EXECUTIONS) MULTIPASS_COUNT

        FROM   V$SQL_WORKAREA_HISTOGRAM

        WHERE  LOW_OPTIMAL_SIZE > 64*1024);   ---- for 64 K optimal size

- V$SQL_WORKAREA_ACTIVE

This view can be used to display the work areas that are active (or executing)  in the instance. Small active sorts (under 64 KB) are excluded from the view.  Use this view to precisely monitor the size of all active work areas and to  determine if these active work areas spill to a temporary segment.

Example :

SELECT TO_NUMBER(DECODE(SID, 65535, NULL, SID)) SID,

       OPERATION_TYPE OPERATION,TRUNC(EXPECTED_SIZE/1024) ESIZE,

       TRUNC(ACTUAL_MEM_USED/1024) MEM, TRUNC(MAX_MEM_USED/1024) "MAX MEM",

       NUMBER_PASSES PASS, TRUNC(TEMPSEG_SIZE/1024) TSIZE

FROM V$SQL_WORKAREA_ACTIVE

ORDER BY 1,2;

SID OPERATION         ESIZE     MEM       MAX MEM    PASS TSIZE

--- ----------------- --------- --------- --------- ----- -------

8   GROUP BY (SORT)   315       280       904         0

8   HASH-JOIN               2995      2377      2430        1   20000

9   GROUP BY (SORT)   34300     22688     22688       0

11  HASH-JOIN              18044     54482     54482       0

12  HASH-JOIN              18044     11406     21406       1   120000

This output shows that session 12 (column SID) is running a hash-join having its work area running in one-pass mode (PASS column). This work area is currently using 11406 KB of memory (MEM column) and has used, in the past, up to 21406 KB of PGA memory (MAX MEM column). It has also spilled to a temporary segment of size 120000 KB. Finally, the column ESIZE indicates the maximum amount of memory that the PGA memory manager expects this hash-join to use. This maximum is dynamically computed by the PGA memory manager according to workload.

When a work area is deallocated—that is, when the execution of its associated SQL operator is complete—the work area is automatically removed from the  V$SQL_WORKAREA_ACTIVE view.

- Note: 148346.1 have some other queries we use to monitor SQL execution memory

3- The Third and last step is tuning the PGA_AGGREGATE_TARGET. In Oracle 9i Release 2 we have 2 new views that help us in this task

- V$PGA_TARGET_ADVICE

- V$PGA_TARGET_ADVICE_HISTOGRAM

By examining these two views, you will be able to determine how key PGA statistics will be impacted if you change the value of PGA_AGGREGATE_TARGET.

To enable automatic generation of PGA advice performance views, make sure the  following parameters are set:

- PGA_AGGREGATE_TARGET

- STATISTICS_LEVEL. Set this to TYPICAL (the default) or ALL; setting this   parameter to BASIC turns off generation of PGA performance advice views.

The content of these PGA advice performance views is reset at instance start-up or when PGA_AGGREGATE_TARGET is altered.   NOTE:  PGA_AGGREGATE can change automatically over time starting with 11g as part of the Automatic Memory Management enhancements available at 11g.   See Note:443746.1 for more details.

V$PGA_TARGET_ADVICE view predicts how the statistics cache hit percentage and  over allocation count in V$PGASTAT will be impacted if you change the value of  the initialization parameter PGA_AGGREGATE_TARGET.

The following select statement can be used to find this information

SELECT ROUND(PGA_TARGET_FOR_ESTIMATE/1024/1024) TARGET_MB,        ESTD_PGA_CACHE_HIT_PERCENTAGE CACHE_HIT_PERC,     ESTD_OVERALLOC_COUNT

FROM   V$PGA_TARGET_ADVICE;

The output of this query might look like the following:

TARGET_MB  CACHE_HIT_PERC ESTD_OVERALLOC_COUNT

---------- -------------- --------------------

63         23             367

125        24             30

250        30             3

375        39             0

500        58             0

600        59             0

700        59             0

800        60             0

900        60             0

1000       61             0

1500       67             0

2000       76             0

3000       83             0

4000       85             0

From the above results we should set the PGA_AGGREGATE_TARGET parameter to a value where we avoid any over allocation, so lowest PGA_AGGREGATE_TARGET value we can set is 375 ( where ESTD_OVERALLOC_COUNT is 0)

After eliminating over-allocations, the goal is to maximize the PGA cache hit percentage, based on your response-time requirement and memory constraints.

V$PGA_TARGET_ADVICE_HISTOGRAM view predicts how the statistics displayed by the performance view V$SQL_WORKAREA_HISTOGRAM will be impacted if you change the value of the initialization parameter PGA_AGGREGATE_TARGET. You can use the dynamic view V$PGA_TARGET_ADVICE_HISTOGRAM to view detailed information on the predicted number of optimal, one-pass and multi-pass work area executions for the set of PGA_AGGREGATE_TARGET values you use for the prediction.