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, $object_policy_statistics is not maintained if DRM
is disabled.
This command will change _gc_policy_time:
Note: NEVER change a hidden parameter without
informing and with the consent of Oracle technical support.
SQL> alter system set
"_gc_policy_time" = 1 scope=spfile
You can display these _gc_policy_time
and related parameters with this query:
set
lines 235
col
Parameter FOR a20
col Instance FOR
a10
col Description FOR a40 word_wrapped
SELECT
a.ksppinm "Parameter",
c.ksppstvl
"Instance",
a.ksppdesc "Description"
FROM
x$ksppi
a,
x$ksppcv b,
x$ksppsv c,
v$parameter p
WHERE
a.indx = b.indx
AND
a.indx = c.indx
AND
p.name(+) =
a.ksppinm
AND
UPPER(a.ksppinm)
LIKE UPPER('%gc_policy%')
ORDER BY
a.ksppinm;
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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