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Oracle Tools for Tuning Memory Contention

Oracle Tips by Burleson Consulting

Memory contention will cause the best-tuned application to perform poorly. If the application constantly has to go to disk to get data dictionary and actual data, then performance will suffer. Remember, the SGA is divided into three major areas: the shared pool, the redo log buffer, and the database buffers. Under ORACLE8, Oracle8i, and Oracle9i, there are these additional areas in the SGA: the large pool (Oracle8), for databases using the multithreaded server, parallel query and rman, and the Java Pool (Oracle8i and Oracle9i) for all databases.

Tuning the Shared Pool

Missing a get on the data dictionary or shared pool area of the SGA is more costly than missing a get on a data buffer or waiting for a redo buffer. Therefore, here we will look at an SQL script that allows the Remote DBA to examine the current status of the data dictionary or shared pool area. This SQL script is shown in Source 13.1.

Notice the following in Source 13.1: First, the script only selects statistics that have been used more than 100 times and where getmisses occurred. Obviously, if the parameter has had no getmisses, it should be satisfactory. (The factor of 100 gets was selected to ensure that the parameter has had enough activity to generate valid statistics.) Also notice that the percentage of misses is automatically calculated and reported for each parameter. If the Remote DBA desires, the percent value could be used to generate a decoded value of RAISE if the percent is greater than 10, or LOWER if the value is less than a predetermined value. An example of this script’s output is shown in Listing 13.2.

SOURCE 13.1 Data dictionary cache report.

REM
REM NAME            : DD_CACHE.SQL
REM FUNCTION     : GENERATE REPORT ON DATA DICTIONARY CACHE
REM                CONDITION
REM USE          : FROM SQLPLUS
REM Limitations      : None
REM Revisions:
REM Date          Modified By     Reason For change
REM 21-AUG-1991     MIKE AULT     INITIAL CREATE
REM 27-NOV-1991     MIKE AULT     ADD % CALCULATION TO REPORT
REM 28-OCT-1992     MIKE AULT     ADD CALL TO TITLE PROCEDURE
REM 21-Jun-1997     MIKE AULT     Updated to ORACLE8
REM 07-nov-2001     MIKE AULT     Tested on 9i, reformatted
REM SET FLUSH OFF
REM SET TERM OFF
SET HEAD ON
SET PAGESIZE 59
SET LINESIZE 79
COLUMN parameter FORMAT A20
COLUMN type FORMAT a11
COLUMN percent FORMAT 999.99 HEADING "%";
COLUMN gets FORMAT 999,999 HEADING 'Gets'
COLUMN getmisses FORMAT 999,999 heading 'Get|Misses'
COLUMN count FORMAT 999,999 heading 'Count'
COLUMN usage FORMAT 999,999 HEADING 'Usage'
ttitle "DATA DICTIONARY CACHE STATISTICS"
SPOOL rep_out/ddcache.lis
SELECT
     parameter,
     type,
     gets,
     getmisses,
     ( getmisses / gets * 100) percent,
      count,
     usage
FROM
     v$rowcache
WHERE
     gets > 100 AND
     getmisses > 0
ORDER BY parameter;
SPOOL OFF

LISTING 13.2 Example output from DD_CACHE script.

Wed Nov 07 page 1
DATA DICTIONARY CACHE STATISTICS

                                           Get
PARAMETER            TYPE          Gets Misses     % Count    Usage
-------------------- ----------- ------ ------ ----- ----- --------
dc_constraints       PARENT       1,004    503 50.10   509      503
dc_files             PARENT         178      2 1.12    11        2
dc_free_extents      PARENT       2,413    825 34.19    30       23
dc_global_oids       PARENT         114     30 26.32    35       30
dc_histogram_defs    PARENT       1,124    165 14.68   166      165
dc_object_ids        PARENT      11,498 1,516 13.18 1,397    1,391
dc_objects           PARENT      12,232 2,696 22.04 2,749    2,746
dc_rollback_segments PARENT         738     12 1.63    18       13
dc_segments          PARENT       7,915 1,070 13.52 1,076    1,068
dc_sequences         PARENT         277    53 19.13    58       53
dc_tablespace_quotas PARENT       3,013      2   .07    24        2
dc_tablespaces       PARENT      10,281      5   .05     6        5
dc_used_extents      PARENT         805    804 99.88   811      801
dc_user_grants       SUBORDINATE 13,576     16   .12    16       14
dc_usernames         PARENT       4,993     14   .28    21       14
dc_users             PARENT      20,200     26   .13    29       26

16 rows selected.

In reviewing this report, check the following:

* Review Count and Usage columns: If Usage is equal to Count, the cache area is being fully utilized.

* If Usage is consistently low compared to Count, consider reducing the INIT.ORA parameter that controls the caches (SHARED_POOL).

* If Count and Usage are nearly equal and the percents are greater than 10, consider increasing the INIT.ORA parameter that controls the caches (SHARED_POOL_SIZE).

Since we are actually concerned only with an aggregate look at the cache area performance, the following query can be substituted into the report to give you an overall health indicator:

SELECT (SUM(getmisses) / SUM(gets)) 'DD CACHE MISS RATIO'
FROM V$ROWCACHE;

This substitution simplifies the report into:

Wed Nov 07 page 1
DATA DICTIONARY CACHE STATISTICS

DD CACHE MISS RATIO:
--------------------
.085476693

The usual guidelines state that, if this ratio gets above 10 percent, take action by increasing the shared pool size parameter; that said, I have not seen this ratio greater than 1 percent on a properly tuned instance, so I would recommend earlier action. In our example instance, therefore, we may need to increase the shared pool size; however, we first need to look at other factors such as frequency of shared pool flushes (if any) and how the pool is actually being used before we just start adding to it.

Advanced Tuning of the Shared Pool

Perhaps one of the least-understood aspects of Oracle Shared Global Area tuning is tuning the shared pool. The generally accepted tuning methodology involves throwing memory into the pool until either the problem goes under or the problem is masked. Here we will examine the shared pool and define a method for tuning it that uses measurement, not guesswork, to drive the tuning methodologies. Numerous scripts for examining the shared pool are provided.

What is the shared pool?

Many people know that the shared pool is a part of the Oracle SGA but little else, so to begin this discussion it’s necessary to answer exactly,What is the shared pool? The shared pool contains several key Oracle performance-related memory areas. If the shared pool is improperly sized, then overall database performance will suffer, sometimes dramatically. Figure 13.1 diagrams the shared pool structure located inside Oracle 8i and 9i SGAs.

Shared pools in Oracle8i and Oracle9i.

As you can see from the structures pictured in Figure 13.1, the shared pool is separated into many substructures. The substructures of the shared pool fall into two broad areas: the fixed-size areas, which, for a given database at a given point in time stay relatively constant in size, and the variable-size areas, which grow and shrink according to user and program requirements.

In Figure 13.1, the areas inside the library caches’ substructure are variable in size, while those outside the library caches (with the exception of the request and response queues used with MTS) stay relatively fixed in size. The sizes are determined based on an Oracle internal algorithm that ratios out the fixed areas based on overall shared pool size, a few of the initialization parameters, and empirical determinations from previous versions. In early versions of Oracle (notably 6.2 and earlier), the dictionary caches could be sized individually allowing a finer control of this aspect of the shared pool. With Oracle 7, the internal algorithm for sizing the data dictionary caches took control from the Remote DBA.

The major difference between the shared pools in Oracle8i and Oracle9i is that any excess memory specified by the SGA_MAX_CACHE parameter and not used in the actual cache and buffer definitions will be placed in the miscellaneous area of the shared pool.

The shared pool is used for objects that can be shared among all users, such as table definitions, reusable SQL (although non-reusable SQL is also stored there), PL/SQL packages, procedures, and functions. Cursor information is also stored in the shared pool. At a minimum, the shared pool must be sized to accommodate the needs of the fixed areas, plus a small amount of memory reserved for use in parsing SQL and PL/SQL statements. If this is not done, ORA-04031 and ORA-07445 errors will result.

See Code Depot for Full Scripts

This is an excerpt from Mike Ault, bestselling author of "Oracle 10g Grid and Real Application Clusters".

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.