Main components of shared pool are library cache (executable forms of SQL cursors, PL/SQL programs, and Java classes.) and the dictionary cache (usernames, segment information, profile data, tablespace information, and sequence numbers. ).The library cache stores the executable (parsed or compiled) form of recently referenced SQL and PL/SQL code. The dictionary cache stores data referenced from the data dictionary. This caches are managed by LRU algorithm to “age out” memory structures that have not been reused over time. Allocation of memory from the shared pool is performed in chunks. This allows large objects (over 5k) to be loaded into the cache without requiring a single contiguous area, hence reducing the possibility of running out of enough contiguous memory due to fragmentation. Starting with 9i The Shared Pool divide its shared memory areas into subpools. Each subpool will have Free List Buckets (containing pointers to memory chunks within the subpool ) and , memory structure entries, and LRU list. This architecture is designed to to increase the throughput of shared pool in that now each subpool is protected by a Pool child latch. This means there is no longer contention in the Shared Pool for a single latch as in earlier versions.
Infrequently, Java, PL/SQL, or SQL cursors may make allocations out
of the shared pool that are larger than 5k then Oracle must search for
and free enough memory to satisfy this request. This operation could
conceivably hold the latch resource for detectable periods of time,
causing minor disruption to other concurrent attempts at memory
allocation. To allow these allocations to occur most efficiently,
Oracle segregates a small amount of the shared pool. This memory is
used if the shared pool does not have enough space. The segregated area
of the shared pool is called the reserved pool which
is also divided into subpools. Smaller objects will not fragment the
reserved list, helping to ensure the reserved list will have large
contiguous chunks of memory. Once the memory allocated from the
reserved list is freed, it returns to the reserved list.
By using automatic shared memory management (ASMM) option available with 10G, which is activated by setting SGA_TARGET parameter with a value greater than 0 and STATISTICS_LEVEL to TYPICAL or ALL, shared pool size is started to be managed by Oracle, under the limits of SGA_TARGET and other SGA components.
After these explanations lets start to explain how to manage shared pool with ASMM.
1- Using Shared Pool Effectively
avoid hard parsing by
substr(sql_text,1,40) “SQL”, count(*) , sum(executions) “TotExecs”;
WHERE executions < 5
GROUP BY substr(sql_text,1,40)
HAVING count(*) > 30
ORDER BY 2;
GROUP BY hash_value
HAVING count(*) > 5;
3- Identify which sqls are using lots of shared memory
substr(sql_text,1,20) “Stmt”, count(*),
GROUP BY substr(sql_text,1,20)
HAVING sum(sharable_mem) > 1426063 –%10 of Shared Pool Size;
This should show if there are similar literal statements, or multiple versions of a statements which account for a large portion of the memory in the shared pool.
4- Identify which allocations causing shared pool to be aged out
SELECT *FROM x$ksmlru
This select returns no more than 10 rows and then erases the contents of the X$KSMLRU table so be sure to SPOOL the output. The X$KSMLRU table shows which memory allocations have caused the MOST memory chunks to be thrown out of the shared pool since it was last queried. This is sometimes useful to help identify sessions or statements which are continually causing space to be requested.
5- Why are there multiple child cursors.
V$SQL_SHARED_CURSOR explains why a particular child cursor is not shared with existing child cursors. Each column identifies a specific reason why the cursor cannot be shared.
FROM V$SQLAREA SA, V$SQL_SHARED_CURSOR SS
AND SA.VERSION_COUNT > 5
ORDER BY SA.VERSION_COUNT ;
6- Monitor Shared Pool sizing operations
You can see the shrinking and growing operations from V$SGA_RESIZE_OPS dynamic view and you can guess why there is need for this operations by focusing the sql at the sizing operation times.
to_char(end_time, ‘dd-Mon-yyyy hh24:mi’) end, oper_type, initial_size,
target_size, final_size from V$SGA_RESIZE_OPS
where component=’shared pool
order by end;
7- Minimum Size of Shared Pool
Current size of the shared pool;
where name=’Shared Pool Size’;
You can use the sizig advices from the view v$shared_pool_advice. This view displays information about estimated parse time in the shared pool for different pool sizes and the sizes range from %10 to %200 of current shared pool size. This can give you idea for sizing SGA and obliquely shared pool by the help of ASMM.
select * from V$SHARED_POOL_ADVICE;
Suggested minimum Shared Pool Size:
set numwidth 20
column cr_shared_pool_size format 999,999,999,999
column sum_obj_size format 999,999,999,999
column sum_sql_size format 999,999,999,999
column sum_user_size format 999,999,999,999
column min_shared_pool format 999,999,999,999
select cr_shared_pool_size, sum_obj_size, sum_sql_size, sum_user_size,
(sum_obj_size + sum_sql_size+sum_user_size)* 1.3 min_shared_pool
from (select sum(sharable_mem) sum_obj_size from v$db_object_cache where type<> 'CURSOR'),
(select sum(sharable_mem) sum_sql_size from v$sqlarea),
(select sum(250*users_opening) sum_user_size from v$sqlarea),
(select to_Number(b.ksppstvl) cr_shared_pool_size from x$ksppi a, x$ksppcv b, x$ksppsv c
where a.indx = b.indx and a.indx = c.indx
and a.ksppinm ='__shared_pool_size' );
You should set the suggested minimum shared pool size to avoid shrinking operation of ASMM
|alter system set shared_pool_size=73M;|
8- How much free memory in SGA is available for shared pool and how to interpret the free memory
First of all find the free memory in shared pool. If you have free memory you should relax but if you don’t have go to the step below
WHERE NAME = ‘FREE MEMORY’
AND POOL = ‘SHARED POOL’;no rows selected
The X$KSMSP view shows the breakdown of memory in the SGA. You can run this query to build trend information on memory usage in the SGA. Remember, the ‘free’ class in this query is not specific to the Shared Pool, but is across the SGA. Dont use the script below when db is under load. Check out Jonathan Lewis’s experiences on this view from here
KSMCHCLS CLASS, COUNT(KSMCHCLS) NUM, SUM(KSMCHSIZ) SIZ,
To_char( ((SUM(KSMCHSIZ)/COUNT(KSMCHCLS)/1024)),’999,999.00′)||’k’ “AVG SIZE”
FROM X$KSMSP GROUP BY KSMCHCLS;
Watch for trends using these guidelines:
a) if ‘free’ memory is low (less than 5mb or so) you may need to
increase the shared_pool_size and shared_pool_reserved_size. You should
expect ‘free’ memory to increase and decrease over time. Seeing trends
where ‘free’ memory decreases consistently is not necessarily a
problem, but seeing consistent spikes up and down could be a problem.
b) if ‘freeable’ or ‘perm’ memory continually grows then it is possible you are seeing a memory bug.
c) if ‘freeabl’ and ‘recr’ memory classes are always huge, this indicates that you have a lot of cursor info stored that is not releasing.
d) if ‘free’ memory is huge but you are still getting 4031 errors, the problem is likely reloads and invalids in the library cache causing fragmentation.
!!!!!!!!!! Note says that this query can hang database on HP platforms
To see the free memory chunks detailed use the script below
KSMCHIDX “SubPool”, ’sga heap(’||KSMCHIDX||’,0)’sga_heap,ksmchcom
decode(round(ksmchsiz/1000),0,’0-1K’, 1,’1-2K’, 2,’2-3K’,3,’3-4K’, 4,’4-5K’,5,’5-6k’,6,’6-7k’,7,’7-8k’,8,‘8-9k’, 9,’9-10k’,'> 10K’) “size”,
count(*),ksmchcls Status, sum(ksmchsiz) Bytes
where KSMCHCOM = ‘free memory’
group by ksmchidx, ksmchcls,
’sga heap(’||KSMCHIDX||’,0)’,ksmchcom, ksmchcls,decode(round(ksmchsiz/1000),0,’0-1K’, 1,’1-2K’, 2,’2-3K’, 3,’3-4K’,4,’4-5K’,5,’5-6k’,6, ‘6-7k’,7,’7-8k’,8,’8-9k’, 9,’9-10k’,'> 10K’);
If you see lack of large chunks it is possible that you can face
with ORA-04031 in near future.
9- Is library_cache or dictionary_cache utilization satisfactory ?
The statistics below is based since the start of the instance. You should take interval statistics to interpret these values for performance issues .
NAMESPACE, PINS, PINHITS, RELOADS, INVALIDATIONS
ORDER BY NAMESPACE;
High invalidations indicates that there is parsing problem with the namespace and high reloads indicates that there is a sizing problem which causes aging out.
SUM(PINHITS)/SUM(PINS) FROM V$LIBRARYCACHE;
low hit ratio is an indication of a sizing or caching problem
PARAMETER, SUM(GETS) , SUM(GETMISSES), 100*SUM(GETS - GETMISSES) /
SUM(GETS) PCT_SUCC_GETS, SUM(MODIFICATIONS) UPDATES
WHERE GETS > 0
GROUP BY PARAMETER;
High updates with low pct_succ_gets can be a clue of performance
problems when accessing that dictionary object. For frequently accessed
dictionary caches, the ratio of total
GETMISSES to total
should be less than 10% or 15%, depending on the application. If this
ratio is higher and every previous control is OK then you should
consider to increase the shared pool size
(SUM(GETS - GETMISSES - FIXED)) / SUM(GETS) “ROW CACHE” FROM V$ROWCACHE;
Low hit ratio is an indication of a sizing problem.
10- Are there any objects candidate for library cache pinning ?
Having objects pinned will reduce fragmentation and changes of encountering the ORA-04031 error. Objects causing a large number of other objects been flushed out from the shared pool are candidates to be pinned into the shared pool using dbms_shared_pool.keep procedure. You can check the x$ksmlru fixed table to see the candidates. This table keeps track of the objects and the corresponding number of objects flushed out of the shared pool to allocate space for the load. These objects are stored and flushed out based on the Least Recently Used (LRU) algorithm. Because this is a fixed table, once you query the table, Oracle will automatically reset the table so first insert the contents to temporary table like below,
|CREATE TABLE LRU_TMP AS SELECT * FROM X$KSMLRU;|
and on regular intervals issue
|INSERT INTO LRU_TMP SELECT * FROM X$KSMLRU;|
Use the LRU_TMP table for analysis. You can use a query below to see more information on candidate code in the library cache.
USERNAME, KSMLRCOM, KSMLRHON, KSMLRNUM, KSMLRSIZ, SQL_TEXT
FROM V$SQLAREA A, LRU_TMP K, V$SESSION S
WHERE KSMLRSIZ > 3000
AND A.ADDRESS=S.SQL_ADDRESS AND A.HASH_VALUE = S.SQL_HASH_VALUE
You can see the candidates to pin from the query below
STORED_OBJECT FORMAT A40;
COL SQ_EXECUTIONS FORMAT 999,999;
SELECT /*+ ORDERED USE_HASH(D) USE_HASH(C) */ O.KGLNAOWN||’.'||O.KGLNAOBJ STORED_OBJECT, SUM(C.KGLHDEXC) SQL_EXECUTIONS
FROM SYS.X$KGLOB O, SYS.X$KGLRD D, SYS.X$KGLCURSOR C
WHERE O.INST_ID = USERENV(’INSTANCE’) AND
D.INST_ID = USERENV(’INSTANCE’) AND
C.INST_ID = USERENV(’INSTANCE’) AND
O.KGLOBTYP IN (7, 8, 9, 11, 12) AND
D.KGLHDCDR = O.KGLHDADR AND
C.KGLHDPAR = D.KGLRDHDL
GROUP BY O.KGLNAOWN, O.KGLNAOBJ
HAVING SUM(C.KGLHDEXC) > 0
ORDER BY 2 DESC;
You should pin objects you find immediatelly after the each restart of instance. You can pin the object by DBMS_SHARED_POOL package like below
|EXECUTE DBMS_SHARED_POOL.KEEP(OWNER.TRIGGER, ‘R’)|
11- Is my Reserved Area sized properly?
An ORA-04031 error referencing a large failed requests indicates the Reserved Area is too fragmented.
free_space for 999,999,999,999 head “TOTAL FREE”
col avg_free_size for 999,999,999,999 head “AVERAGE|CHUNK SIZE
col free_count for 999,999,999,999 head “COUNT”
col request_misses for 999,999,999,999 head “REQUEST|MISSES
col request_failures for 999,999,999,999 head “REQUEST|FAILURES”
col max_free_size for 999,999,999,999 head “LARGEST CHUNK”
select free_space, avg_free_size, free_count, max_free_size, request_misses, request_failures
The reserved pool is small when:
REQUEST_FAILURES > 0 (and increasing)
The DBA should Increase shared_pool_reserved_size and shared_pool_size together.
It is possible that too much memory has been allocated to the
REQUEST_MISS = 0 or not increasing
FREE_MEMORY = > 50% of shared_pool_reserved_size minimum
The DBA should Decrease shared_pool_reserved_size
You should also use hidden and unsupported parameter “_shared_pool_reserved_pct” to control reserved pool. This parameter controls the allocated percentage of shared pool for reserved pool. By default it is %5 of the shared pool and if you use ASMM for memory management you can set this value higher like 10 to allocate reserved pool dynamically. When you set the parameter you will see the shared_pool_reserved_size parameter will be adjusted to the new setting.
The parameter can not be modified when instance is started. You can use the query below to see the current value
a.ksppinm “Parameter”, b.ksppstvl “Session Value”, c.ksppstvl “Instance
from sys.x$ksppi a, sys.x$ksppcv b, sys.x$ksppsv c
where a.indx = b.indx and a.indx = c.indx
and a.ksppinm = ‘_shared_pool_reserved_pct’;
12-Is there any fragmentation in shared pool?
The primary problem that occurs is that free memory in the shared pool becomes fragmented into small pieces over time. Any attempt to allocate a large piece of memory in the shared pool will cause large amount of objects in the library cache to be flushed out and may result in an ORA-04031 out of shared memory error. But how to understand the fragmentation ?
|select * from x$ksmlru where ksmlrsiz > 5000;|
After finding the result you should do the followings to correct fragmentation
sql_text from v$sqlarea
where command_type=47 — command type for anonymous block
and length(sql_text) > 500;
Fallacies about solving shared pool fragmentation
13- Using related database parameters
You must be sure that the shared pool is large enough for the work load otherwise performance will be badly affected and ORA-4031 eventually signalled.
V$SYSSTATview. This statistic counts the number of times a parse call found a cursor in the session cursor cache. If this statistic is a relatively low percentage of the total parse call count for the session, then consider setting
SESSION_CACHED_CURSORSto a larger value. Steve Adams also wrote usefully queries to find the usage and the maximum cacheable cursors.session_cursor_cache.sql
This was a long article and if you see anything wrong or suspicious please feel free to comment for correction
All of the queries are tested on Oracle 10.2.0.3 for Windows
Code Depot of The Queries
(All scripts are taken from metalink notes and official documentation)
Oracle® Database Performance Tuning Guide 10g Release 2 (10.2)
Note:396940.1 Troubleshooting and Diagnosing ORA-4031 Error
Note:146599.1 Diagnosing and Resolving Error ORA-04031
Note:61623.1 Resolving Shared Pool Fragmentation In Oracle7
Note:62143.1 Understanding and Tuning the Shared Pool
Note:1012047.6 How To Pin Objects in Your Shared Pool
Note:274496.1 ora-7445 and ora-4031 in 22.214.171.124 and 10g if SESSION_CACHED_CURSORS is used
www.ixora.com Oracle Advanced Performance Tuning Scripts
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