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@xxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxx>
Sent: 03 March 2020 17:07
To: Noveljic Nenad; Jonathan Lewis; ORACLE-L (oracle-l@xxxxxxxxxxxxx)
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@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> On Behalf
Of Noveljic Nenad
Sent: Dienstag, 3. März 2020 16:33
To: jonathan@xxxxxxxxxxxxxxxxxx; ORACLE-L (oracle-l@xxxxxxxxxxxxx)
<oracle-l@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> On Behalf
Of Jonathan Lewis
Sent: Dienstag, 3. März 2020 16:03
To: ORACLE-L (oracle-l@xxxxxxxxxxxxx) <oracle-l@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>
Sent: 03 March 2020 14:52
To: ORACLE-L (oracle-l@xxxxxxxxxxxxx)
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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