ORACLE9i_性能调优基础三(Tuning the Shared Pool)

Objectives

1.Determine the size of an object and pin it in the shared pool

2.Tune the shared pool reserved space

3.Describe the user global area(UGA) and session memory considerations

4.Measure the library cache hit ratio

5.List other tuning issues related to the shared pool

6.Measure the dictionary cache hit ratio

7.set the large pool

Shared pool Contents

The Library Cache

1.Used to store SQL statements and PL/SQL blocks that are no be shared by users

2.Managed by a least recently used(LRU) algrithm

3.Used to prevent statements reparsing

4.Reports error ORA-04031 if the shared pool is not of free memory



select * from v$sgastat;

SQL Sharing Criteria

Oracle automatically determines whether a SQL statement or PL/SQL block being issued is identical to another statement currently in the shared pool

1.The text of the statement issued is compared to existing statements in the shared pool

2.The text of the statement is hashed . If there is no matching hash value. then the SQL statement does not currently exist in the shared pool. and a hard parse is performed

3.if there is a matching hash value for an existing SQL statement in the shared pool, then Oracle compares the text of the matched statement to the text of the statement hashed to see if they are identical . The text of the SL
statements or PL/SQL blocks must be identical , character for character, including spaces,case, and comments .

Usually , SQL statements that differ only in literals cannot use the same shared SQL area.For example, the following SQL statements do not resolve to the same SQL area:

SELECT COUNT(1) FROM employees WHERE manager_id=121;

SELECT COUNT(1) FROM employees WHERE manager_id=247;

The only exception to this rule is when the parameter CURSOR_SHARING has been set to SIMILAR or FORCE.

Similar statements can share SQL areas when the CURSOR_SHARING parameter is set to SIMILAR or FORCE.The cost and benefits involved in using CURSOR_SARING are explained later in this section.

4.The objects referenced in the issued statement are compared to the referenced objects of all existing satements in the shared pool to ensure that they are identical.

References to scema objects in the SQL statements or PL/SQL blocks must resolve to the same object in the same schema.For example if two users eac issue the following SQL statement:

SELECT * FROM employees

and they each have their own employees table ,then this statements is not considered identical ,because the statement references different tables for each user.

5.Blind variables in the SQL statements must match in name, datatype, and length.

For example , the following statements cannot use the same shared SQ erea, bacause the bind variable names differ:

SELECT * FROM employees where department_id=:department_id;

SELECT * FROM employees where department_id=:dept_id;

Many Oracle products ,such as ORACLE Forms and the precompilers ,convert the SQL before passing statements to the database. Characters are uniformly changed to uppercase, white space is compressed, and bind variables are renamed
so that a consistent set of SQL statements is produced.

6.The session's environment must be identical .For example, SQL statements must be optimized using te same optimization goal.

Use Bind Variables

QL> show parameter cursor_sharing;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cursor_sharing                       string      EXACT

SQL> select * from v$sysstat where name like '%parse count (hard)%';

STATISTIC# NAME                                CLASS      VALUE    STAT_ID
---------- ------------------------------ ---------- ---------- ----------
331 parse count (hard)                     64       1904  143509059

SQL> select * from ta where id=2;

no rows selected

SQL> select * from v$sysstat where name like '%parse count (hard)%';

STATISTIC# NAME                                CLASS      VALUE    STAT_ID
---------- ------------------------------ ---------- ---------- ----------
331 parse count (hard)                     64       1905  143509059

SQL> select * from ta where id=3;

no rows selected

SQL> select * from v$sysstat where name like '%parse count (hard)%';

STATISTIC# NAME                                CLASS      VALUE    STAT_ID
---------- ------------------------------ ---------- ---------- ----------
331 parse count (hard)                     64       1906  143509059

</pre><pre class="sql" name="code"> select namespace, gets, gethits,pins, pinhits,reloads, invalidations from v$librarycache;

NAMESPAC       GETS    GETHITS       PINS    PINHITS    RELOADS INVALIDATIONS
-------- ---------- ---------- ---------- ---------- ---------- -------------
SQL AREA       6665       3078      68118      63401       1090            14
TABLE/PR       6939       4414      19214      14724        972             0
OCEDURE

BODY            751        700        977        866         57             0
TRIGGER          86         62        323        268         31             0
INDEX           207         37       1295       1009         22             0
CLUSTER         244        235        674        656          9             0
OBJECT            0          0          0          0          0             0
PIPE              0          0          0          0          0             0
JAVA SOU          0          0          0          0          0             0

NAMESPAC       GETS    GETHITS       PINS    PINHITS    RELOADS INVALIDATIONS
-------- ---------- ---------- ---------- ---------- ---------- -------------
RCE

JAVA RES          0          0          0          0          0             0
OURCE

JAVA DAT          0          0          0          0          0             0
A

11 rows selected.

Diagnostic Tools

SQL>SELECT SQL_TEXT FROM V$SQLAREA WHERE EXECUTIONS<5 ORDER BY UPPER(SQL_TEXT);
SELECT SQL_TEXT, PARSE_CALLS,EXECUTIONS FROM v$SQLAREA ORDER BY PARSE_CALLS;  --PARSE_CALLS 如果和EXECUTIONS相差不大 ，解析次数多

Using SP Effectively

Shared Cursors

Library Cache Staticstics

SQL>  select namespace,gethitratio,pinhitratio,reloads,invalidations from v$librarycache;

NAMESPACE       GETHITRATIO PINHITRATIO    RELOADS INVALIDATIONS
--------------- ----------- ----------- ---------- -------------
SQL AREA         .366623711  .925849382        661             0
TABLE/PROCEDURE    .5681594  .655021834        644             0
BODY             .961231471  .949468085         21             0
TRIGGER                  .8  .974110032          4             0
INDEX             .25974026  .037974684         19             0
CLUSTER          .935483871  .961165049          4             0
OBJECT                    1           1          0             0
PIPE                      1           1          0             0
JAVA SOURCE               1           1          0             0
JAVA RESOURCE             1           1          0             0
JAVA DATA                 1           1          0             0

11 rows selected.

查询正在跑的SQL

SQL> select sql_text,users_executing,executions,loads from v$sqlarea;

Sharing Cursors:

Values for CURSOR_SHARING are: Exact Similar, Force

Library Cache Reloads

pins, reloads 这些都是累计值,不要直接用。隔一段时间执行两次 取增量值

Sizeing Library Cache

1.Define the global space necessary for stored objects;

2.Define the amount of memory used by the usual SQL statements

3.Resesrve space for large memory requirements to avoid misses and fragmentation.

4.Pin frequently used objects

5.Covert large anonymous PL/SQL blocks into small anonymous blocks calling packaged functions

Shared Pool Advisory

SQL> select shared_pool_size_for_estimate as pool_size ,estd_lc_size,estd_lc_time_saved from v$shared_pool_advice;

POOL_SIZE ESTD_LC_SIZE ESTD_LC_TIME_SAVED
---------- ------------ ------------------
64            8                308
72           15                314
80           22                314
88           29                314
96           34                314
104           34                314
112           34                314
120           34                314
128           34                314
136           34                314
144           34                314

Cached Execution Plans

1.The Oracle server preservers the execution plan of a cached SQL statement in memory

2.When the SQL statement ages out of the library cache, the corresponding cached execution plan is removed

3.You can use the cached plans to idagnose query performance.

Views sql_plan

You can use the v$sql_plan dynamic performance view to view the catual execution plan information for cached cursors

select operation , object_owner, object_name, cost FROM v$sql_plan ORDER BY hash_value;

The v$sql view has a column, plan_hash_value which references the hash_value columen of v$sql_plan.

The column information is a hash value built from the corresponding execution plan

The column can be used to compare cursor plans the same way the hash_value column is used to compare cursor SQL texts.

local Space Allocation

SQL> select sum(sharable_mem) from v$db_object_cache;

SUM(SHARABLE_MEM)
-----------------
8483468

SQL> select sum(sharable_mem) from v$sqlarea where executions>5;

SUM(SHARABLE_MEM)
-----------------
2764410

The Reserved Pool

Although Oracle breaks down very large requets for memory into smaller chunks , on some systems there might till be a requirement to find a contiguous chunk (for example , over 5KB) of memory(The default minimum reservded pool allocation is 4,400 bytes)

If there is not enough free space in the shared pool , then Oracle must searc for and free enough memory to satify this request . This operation could conceivably hold the latch resource for detectable periods of time, causing minor disruption to other concurrent
attempts at memory alloction.

Hence, Oracle internally reserves a samll memory area in the shared pool that can be used if the shaared pool does not have enough space. This reserved pool makes allocation of arge chunks more efficient

By default, Oracle onfigures a small reserved pool.This memory can be used for operatins such a PL/SQL and trigger compilation or for temporary sapce while laoding Java objects.Afte the memory allocated form reserved pool is freed ,it returns to the reserved
pool

You probably will not need to change the default amount of space oracle reserves

However if necessary, the reserved pool size can ben changed by setting the SHARED_POOL_RESERVED_SIZE initialization parameter.This parameter sets aside space in the shared pool for unusually large allocations

For large allocations , Oracle attempts to allocate space in the shared pool in the following order:

1.from the unreserved part of the shared pool.

2.From the reserved pool.If there is not enough space in the unreserved part of the shared pool,then Oracle checks whether the reserved pool has enoug space.

3.From memory.If there is not enough space i the unreserved and reserved parts of the shared pool,then Oracle attempts to free enough memory for the allocation.It thn retries the unreserved and reserved parts of the shared pool.

Large Mem Requirements

SQL> select free_space, requests, request_misses, request_failures from v$shared_pool_reserved;

FREE_SPACE   REQUESTS REQUEST_MISSES REQUEST_FAILURES
---------- ---------- -------------- ----------------
3619096          0              0                0

Tuning SP Reserved Space

1.Diagnostic tools for tuning:

a.The v$shared_pool_reserved dictionary view

b.The supplied aborted_request_threshold procedure in the dbms_shared_pool package

2.Guidelines : Set the SHARED_POOL_RESERVED_SIZE parameter

Tuning SP Reserved Space

1.When SHARED_POOL_RESERVED SIZE IS Too Small

The reserved pool is too small when the value for REQUEST_FAILURES is more than zero and increasing.To resolve this, increase the value for the SHARED_POOL_RESERVED_SIZE and SHARED_POOL_SIZE accordingly.The settings you select for these parameters deped
on your system's SGA size consraints

Increasing the value of SHARED_POOL_RESERVED_SIZE increases the amount of memory available on the reserved list without having an effect on users who do not allocate memory from the reserved list

2.When SHARED_POOL_SIZE is Too SMALL

The v$SHARED_POOL_RESERVED fixed view can also indeicate when the value for SHARED_POOL)SIZE is too small. This can ben case if REQUEST_FAILURES is greater than zero and increasing

If you have enabled the reserved list, then decrease the value for SHARD_POOL_RESERVED_SIZE, If you have not enabled the reserved list, then you could increase SHARED_POOL_SIZE.

3.When SHARED_POOL_RESERVED_SIZE is Too Large

Too much memory migt have been allocated to the reserved list if :

a.REQUEST_MISSES is zero or not increasing

b.FREE_MEMORY is greater than or equal to 50% of SHARED_POOL_RESERVED_SIZE minimum

if either of these conditions is ture, then decrease the value for SHARED_POOL_RESERVED_SIZE

Keeping Large Objects

ALTER SYSTEM FLUSH SHARED_POOL; //清空SHARED_POOL ，已PIN的不影响

Oter Parameters

1.OPEN_CURSORS

2.CURSOR_SPACE_FOR_TIME

3. SESSION_CACHED_CURSORS

4.CURSOR_SHARING

Tuning the DD Cache

1.Use v$rowcache to obtain information about the data dictionary cache.

a.Content:Definitions of dictionary objects

b.Terminology:

gets:Number of requests on objects

getmisses: Number of requests resulting in cache misses

c.Tuning:Avoid dictionary cache misses

Measuring the Statistics

Large Pool

SQL> select * from v$sgastat where POOL like 'large%';

POOL         NAME                            BYTES
------------ -------------------------- ----------
large pool   PX msg pool                   1074000
large pool   free memory                   3120304