Flushing Shared Pool Does Not Slow Its Growth

I’m still working on resolving the issues caused by bug 13914613.

Oracle support recommended that we apply a parameter change to resolve the issue but that change requires us to bounce the database  and I was looking for a resolution that does not need a bounce.  The bug caused very bad shared pool latch waits when the automatic memory management feature of our 11.2.0.3 database expanded the shared pool.  Oracle support recommending setting _enable_shared_pool_durations=false and I verified that changing this parameter requires a bounce.  It is a big hassle to bounce this database because of the application so I thought that I might try flushing the shared pool on a regular basis so the automatic memory management would not need to keep increasing the size of the shared pool.  The shared pool was growing in size because we have a lot of SQL statements without bind variables.  So, I did a test and in my test flushing the shared pool did not slow the growth of the shared pool.

Here is a zip of the scripts I used for this test and their outputs: zip

I set the shared pool to a small value so it was more likely to grow and I created a script to run many different sql statements that don’t use bind variables:

spool runselects.sql

select 'select * from dual where dummy=''s'
||to_char(sysdate,'HHMISS')||rownum||''';'
from dba_objects;

spool off

@runselects

So, the queries looked like this:

select * from dual where dummy='s0818111';
select * from dual where dummy='s0818112';
select * from dual where dummy='s0818113';
select * from dual where dummy='s0818114';
select * from dual where dummy='s0818115';
select * from dual where dummy='s0818116';
select * from dual where dummy='s0818117';

I ran these for an hour and tested three different configurations.  The first two did not use the _enable_shared_pool_durations=false setting and the last did.  The first test was a baseline that showed the growth of the shared pool without flushing the shared pool.  The second test including a flush of the shared pool every minute.  The last run included the parameter change and no flush of the shared pool.  I queried V$SGA_RESIZE_OPS after each test to see how many times the shared pool grew.  Here is the query:

SELECT OPER_TYPE,FINAL_SIZE Final,
to_char(start_time,'dd-mon hh24:mi:ss') Started, 
to_char(end_time,'dd-mon hh24:mi:ss') Ended 
FROM V$SGA_RESIZE_OPS
where component='shared pool'
order by start_time,end_time;

Here are the results.

Baseline – no flush, no parameter change:

OPER_TYPE       FINAL STARTED         ENDED
--------- ----------- --------------- ---------------
GROW      150,994,944 18-jun 05:03:54 18-jun 05:03:54
GROW      134,217,728 18-jun 05:03:54 18-jun 05:03:54
STATIC    117,440,512 18-jun 05:03:54 18-jun 05:03:54
GROW      167,772,160 18-jun 05:04:36 18-jun 05:04:36
GROW      184,549,376 18-jun 05:47:38 18-jun 05:47:38

Flush every minute, no parameter change:

OPER_TYPE       FINAL STARTED         ENDED
--------- ----------- --------------- ---------------
GROW      134,217,728 18-jun 06:09:15 18-jun 06:09:15
GROW      150,994,944 18-jun 06:09:15 18-jun 06:09:15
STATIC    117,440,512 18-jun 06:09:15 18-jun 06:09:15
GROW      167,772,160 18-jun 06:09:59 18-jun 06:09:59
GROW      184,549,376 18-jun 06:22:26 18-jun 06:22:26
GROW      201,326,592 18-jun 06:42:29 18-jun 06:42:29
GROW      218,103,808 18-jun 06:47:29 18-jun 06:47:29

Parameter change, no flush:

OPER_TYPE        FINAL STARTED         ENDED
--------- ------------ --------------- ---------------
STATIC     117,440,512 18-jun 07:16:09 18-jun 07:16:09
GROW       134,217,728 18-jun 07:16:18 18-jun 07:16:18

So, at least in this test – which I have run only twice – flushing the shared pool if anything makes the growth of the shared pool worse.  But, changing the parameter seems to lock it in.

– Bobby

About Bobby

I live in Chandler, Arizona with my wife and three daughters. I work for US Foods, the second largest food distribution company in the United States. I have worked in the Information Technology field since 1989. I have a passion for Oracle database performance tuning because I enjoy challenging technical problems that require an understanding of computer science. I enjoy communicating with people about my work.
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