Re: shared pool evictions

From: Jonathan Lewis <>
Date: Tue, 3 Mar 2020 15:03:04 +0000
Message-ID: <LNXP265MB15620933608520F88A772AE5A5E40_at_LNXP265MB1562.GBRP265.PROD.OUTLOOK.COM>

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 ?

Jonathan Lewis

From: <> on behalf of Noveljic Nenad <> Sent: 03 March 2020 14:52
To: ORACLE-L ( 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 ;

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?



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-- Received on Tue Mar 03 2020 - 16:03:04 CET

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