Re: shared pool evictions

No, it's not new.
Reading Tanel's description has reminded me: it's on page 189 of Oracle Core.

"... we have a total of 12,085 recreatable chunks, of these 3,921 are recurrent (rcr) and 8,164 are transient (trn) – the transient ones appear before the separator, the recurrent after the separator. This split is vaguely similar in concept to the mid-point (or cold_head) for the buffer cache – when a new item is inserted into the LRU list it is attached to the head of the transient list; if it is used more than once then it will be moved to the head of the recurrent list – so the transient list ends up with all the objects which aren’t interesting and aren’t used much, and all the interesting stuff that is potentially more interesting ends up in recurrent list."

Regards
Jonathan Lewis


________________________________________
From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Sent: 03 March 2020 17:15
To: Noveljic Nenad
Subject: Re: shared pool evictions


Maybe a new algorithm that is aggressive about aging out cursors that have reached end of fetch, and closed and haven't been re-used very promptly.
It would make sense to do that if you wanted to be sure that it was going to be quick and efficient to switch memory from the shared pool to the buffer cache.  So maybe there's some facor relating to how long the database has been up or the session had been connected.

Regards
Jonathan Lewis

________________________________________
From: Noveljic Nenad <nenad.noveljic_at_vontobel.com>
Sent: 03 March 2020 17:07
To: Noveljic Nenad; Jonathan Lewis; ORACLE-L (oracle-l_at_freelists.org)
Subject: RE: shared pool evictions

The following SQL aged out within a minute when executed from my client (I added a hint to generate a new cursor):

SELECT /*nenad */ INDEX_NAME, CONSTRAINT_TYPE FROM USER_CONSTRAINTS WHERE TABLE_NAME = '...' AND INDEX_NAME IS NOT NULL

However, the query has remained longer in the shared pool when executed by the application.

This means that the relevant difference is probably neither in the query itself nor in the data model, but it's hidden elsewhere. I'll keep looking.

Best regards,

Nenad

https://nenadnoveljic.com/blog/


-----Original Message-----
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Noveljic Nenad
Sent: Dienstag, 3. März 2020 16:33
To: jonathan_at_jlcomp.demon.co.uk; ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: RE: shared pool evictions

The evictions aren't limited to dummies. Very complex queries with subqueries, window functions etc. are affected too, including your example with a single-row-query, pk and a constant.

Actually the queries disappear from the shared pool within a minute.

I'll try to identify the relevant difference between the survivors and losers.

Thanks,

Nenad

https://nenadnoveljic.com/blog/


-----Original Message-----
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Jonathan Lewis
Sent: Dienstag, 3. März 2020 16:03
To: ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: Re: shared pool evictions


First generate a hypothesis:

H1: Oracle has developed an algorithm that tries to identify garbage that should never have been sent to the database and puts it at the top of list for getting chucked out of the SGA at the earliest possible moment to minimise the threat of memory fragmentation.

Devise a test that will prove the hypothesis wrong:  Left as exercise.

Maybe there's some memory related detail that pushes things to the flush end of the LRU - viz: a statement that uses less than X Bytes in the libnrary cache is a fragmentation threat and likely to be garbage (and cheap to recreate if it wasn't).

What happens if the query is : select 1 from single-row-table where pk = constant;


----

More seriously - how much "like" the example are the statement that are evicted; what does the most complex one you can identify look like ?

Regards
Jonathan Lewis




________________________________________
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Noveljic Nenad <nenad.noveljic_at_vontobel.com>
Sent: 03 March 2020 14:52
To: ORACLE-L (oracle-l_at_freelists.org)
Subject: shared pool evictions

The SQL statements like the one below are getting evicted from the shared pool within a couple of minutes on a 18c database:

select /* nenad */ 1 from dual ;

The parameters are set as follows:

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
sga_target                           big integer 10G
shared_pool_size                     big integer 0

The size of the shared pool is ~1.6 GB:
select name,bytes/1024/1024/1024 gb from v$sgainfo ;
NAME              GB
Shared Pool Size  1.6875

There weren't any recent resize operations.

All statements are taking around 25% shared pool space in total:
select ( sum(sharable_mem)+sum(persistent_mem)+sum(runtime_mem) )/1024/1024 mb
  from v$sql ;
478.27296257019

Since there are some statements residing a little bit longer in the shared pool, for which there aren't any open cursors, we can exclude "alter system flush shared_pool" being the source of the problem, I think.

Any ideas how to approach this systematically?

Thanks,

Nenad

https://nenadnoveljic.com/blog/

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