Re: ASMM - resizing triggers/thresholds

Two possibilities -

1. as you suggest, a test is not made immediately after the flush shared pool, with the result that the shared pool has started to fill by the time the test is run and the reparsing that occurs persuades Oracle that a shrink would be a bad idea.

or maybe

b) when you flush the shared_pool items that are pinned are not flushed, and if you have a lot of pinned items scattered across the shared pool then you may have some items in every granule so that granules can't be re-allocated. (But I think in this case you might see "deferred" shrink/grow reported in v$sga_resize_ops / v$memory_resize_ops. If the whole granule can't be re-allocated you may still see buffer cache activity in the shared_pool memory with v$sgastat showing memory with the name 'KGH: NO ACCESS'.

If you can flush the shared pool can't you follow up with a manual resize of the db_cache_size ? As a test, if nothing more ?

Regards
Jonathan Lewis

Hi Jonathan,

thank you for the ideas!

Maybe this mechanism is not triggered by flushing the shared pool. Do you think that's possible?

Regards

--
Martin Klier // Performing Databases GmbH
Managing Partner // Senior DB Consultant
Oracle ACE Director

martin.klier_at_performing-db.com // https://www.performing-databases.com



________________________________


Von: "jonathan" <jonathan_at_jlcomp.demon.co.uk>
An: "Oracle-L Freelists" <oracle-l_at_freelists.org>, "Martin Klier" <martin.klier_at_performing-db.com>
Gesendet: Samstag, 2. Februar 2019 15:03:43
Betreff: Re: ASMM - resizing triggers/thresholds
Martin,

If you look for the hidden parameters like "%memory_broker%" that gives you some clues.
12.2.0.1

NAME                                          SES_VAL
--------------------------------------------- ------------------------------


_automemory_broker_interval                   3

_memory_broker_log_stat_entries               5

_memory_broker_marginal_utility_bc            12

_memory_broker_marginal_utility_sp            7

_memory_broker_shrink_heaps                   15

_memory_broker_shrink_java_heaps              900

_memory_broker_shrink_streams_pool            900

_memory_broker_shrink_timeout                 60000000


_memory_broker_stat_interval                  30





I believe MMAN handles the memory management - so you could enable wait state tracing on that process in a sandbox.

I think there's a calculation every 3 seconds to determine whether or not memory allocations should change, although strangely the parameters above suggest both 3 seconds and 30 seconds . The former apparently is for AMM while the latter is for ASMM.

The marginal_utility suggests to me that the calculation involves the shared pool / buffer cache advice regarding time saved by changing memory size (a variant of the details shown in the AWR), with a requirement (perhaps) of a minimum improvement (12%, 7%) in performance before a change will be triggered.

There is a parameter _memory_management_tracing, default 0, that might enable tracing of the feature(s).

Regards
Jonathan Lewis







________________________________________


From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Martin Klier - Performing Databases GmbH <martin.klier_at_performing-db.com>
Sent: 31 January 2019 10:00:35
To: Oracle-L Freelists
Subject: ASMM - resizing triggers/thresholds

Hi listers,

how and when are memory areas like DB cache and shared pool being resized, when the instance runs in Automatic Shared Memory Management?

In my case, I have a system that has had some shared pool eating (child cursor issue, blowing up the library cache), which was resolved. Now we have a de-facto undersized buffer cache and low library cache consumption, but I can't see any resizing taking place. I have SGA max size = SGA Target and no minimum values for any component here.

Platform: Linux, Oracle EE 12.2.0.1

It would not be a big problem to restart the instance at some point, but this may be different for other cases. So my interest is more educational. :)
In short: Where are the thresholds, what does trigger the resizing, how often is it scanned, when does it happen?

Thanks in advance!
--
Martin Klier // Performing Databases GmbH
Managing Partner // Senior DB Consultant
Oracle ACE Director

martin.klier_at_performing-db.com // https://www.performing-databases.com

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