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Re: Migrating 8i OPS to RAC 9.2

From: K Gopalakrishnan <>
Date: Mon, 23 Jun 2003 06:07:01 -0700
Message-ID: <>


I am not aware any of the documentations which explains this cache fusion to ping pong feature(!). There are 3 components in the typical CR prosessing..

Let us assume there is a resource R which is mastered by the instance A and owned by the instance B on X mode. And also we assume the resource R is requested by Instance C. In this case the requester enquires the status of that resource to the master database and got to know that is owned by instance B. So now it is Instance B's respoisibility to constuct the CR and send it to Instnace C.

Here, there is something called light work rule (X$KCLCRST.LIGHT?) which decides the CR construction and block transfer over interconnect
(again the CR processing for S to N locks are different based on the
setting of _cr_grant_local parameter) or thru the disk transfer.  

Basically the current holder of the resource maintains a fairness counter, which is incremented every time it sends CR copy over the interconnect to the requester and there is a threshold for the number of CR copies created for that resource. Once the ceiling is hit, instead of creating the CR copies, the LMD simply downconverts the lock to NULL and informs the convertion to Intance A.

In simple terms it is like a normal OPS ping. The owner downgrades the lock and the requester reads from the disk after getting approval from the lock master. The threshold is controlled by the parameter _fairness_threshold and IIRC that defaults to 4 or 5. So every 6th (or 5th) CR request will most of the times results in a PING and I have seen good performance improvements in most of the RAC databases by changing this parameter.

Best Regards,
K Gopalakrishnan

(Currently I am in a country (for a week) where I have very limited
access to the internet , So I may not be able to reply if you have any more questions And I don't have any oracle database/documentation to test/verify. So please take the advise with a pinch of salt !)

Please see the official ORACLE-L FAQ:
Author: K Gopalakrishnan

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Received on Mon Jun 23 2003 - 08:07:01 CDT

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