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Don Burleson Blog 







_gc_policy_time tips

Oracle Database Tips by Donald BurlesonMay 25, 2016

Question: What does the _gc_policy_time parameter do?  Can you show an example of using _gc_policy_time?

Answer:  The hidden parameter _gc_policy_time controls how often the queue is checked to see if the re-mastering must be triggered or not with a default value of 10 minutes. dynamic re-mastering of the resources in a RAC database.

The undocumented parameter _gc_policy_time is related to a column in the fixed table x$object_policy_statistics.  The _gc_policy_time is the sum of the last 3 columns (sopens,xopens,xfers) and it decides whether the object will be considered  for the dynamic resource manager (DRM) ( using _gc_policy_minimum).  The duration of the statistics are controlled by _gc_policy_time (and the default is 10 minutes).

See these important related notes on the RAC dynamic resource manager (DRM).

The RAC _gc_policy_time: is related to the Oracle10g _gc_affinity_time parameter, and has a default of _gc_policy_time=10, expressed in minutes.    Interval in minutes in which object statistics are evaluated and decisions are made to establish or dissolve affinity or read-mostly locking. When set to zero, object affinity and read-mostly policies are disabled in DRM.

Following parameters affect dynamic re-mastering due to resource affinity : _gc_policy_limit : If an instance opens 50 more opens on an object then the other RAC instance (controlled by the _gc_policy_limit hidden parameter), then that object is a candidate for remastering. That object is queued and LMD0 reads the queue and initiates GRD freeze. LMON performs reconfiguration of buffer cache locks working with LMS processes.

All these buffer changes are visible in LMD0 and LMON trace files.

The _gc_policy_time controls how often the queue is checked to see if the re-mastering must be triggered or not with a default value of 10 minutes. We also see these related undocumented parameters:

The _gc_policy_minimum parameter is defined as “minimum amount of dynamic affinity activity per minute” to be a candidate for re-mastering. It defaults to 2500 and I think, it is lower in a busy environment.

To disable RAC DRM completely, set _gc_policy_limit and _gc_policy_minimum to much higher value, say 10Million.  Setting the parameter _gc_policy_time to 0 will completely disable DRM, but that also means that you can not manually remaster objects. Further, $object_policy_statistics is not maintained if DRM is disabled.

This command will change _gc_policy_time:  Note:  NEVER change a hidden parameter without informing and with the consent of Oracle technical support.

SQL> alter system set "_gc_policy_time" = 1 scope=spfile

You can display these _gc_policy_time and related parameters with this query:

set lines 235

col Parameter   FOR a20
col Instance    FOR a10
col Description FOR a40 word_wrapped

   a.ksppinm  "Parameter",
   c.ksppstvl "Instance",      
   a.ksppdesc "Description"

   x$ksppi     a,
   x$ksppcv    b,
   x$ksppsv    c,
   v$parameter p

   a.indx = b.indx

   a.indx = c.indx  

AND = a.ksppinm  

   UPPER(a.ksppinm) LIKE UPPER('%gc_policy%')


Parameter            Instance   Description
——————–               ———-      —————————————-
_gc_policy_minimum   20         dynamic object policy minimum activity  per minute

_gc_policy_time      4          how often to make object policy  decisions in minutes



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