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Re: Interpreting global cache waits

From: Michael Boligan <michael.boligan.b_at_bayer.com>
Date: Thu, 28 Oct 2004 14:53:48 -0400
Message-ID: <OFB4BE9132.2CE3D4EA-ON85256F3B.00665C99-85256F3B.0067CCAF@bayer.com>


Alec,

        In version 8i (not sure about 9 or 10), if you are using OPS for anything other than failover (only 1 instance running at a time), then you have to very carefully design your application. It is very expensive to handle locking and latching across nodes. Read Oracle Parallel Server concepts, chapter 5 Parallel Cache management (the cost of locks and coordination of locking mechanisms by DLM are 2 good sections). The 2 solutions you mention may help to alleviate some of the contention, but if the application isn't specifically designed for this (minimize users updating the same table from more than 1 instance), you are fighting the laws of physics.

HTH,
Mike

"Finkelstein, Alec" <AFinkelstein_at_lifelinesys.com> Sent by: oracle-l-bounce_at_freelists.org
10/28/2004 09:51 AM
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Subject
Interpreting global cache waits

Hello,

We have 2-node OPS cluster (8.1.7.4), mostly OLTP. The system is (predictably) susceptible to high CPU saturations. From the 10046 traces I see PCM lock contention for indexes of hot (high updates/inserts tables):

global cache lock null to x
buffer busy due to global cache
global cache lock open x

I'm planning to tackle this by doing hash partitioning and splitting the sequence ranges on the nodes. But my question is as follows: how do I prove that the "problem" is the system being starved of CPU / DLM resources (ie not storage)? That is to say, are the waits indication of storage waits, slow cluster interconnect, or saturated CPUs? E.g. when DLM is waiting to convert consistent read PCM lock to exclusive ("global cache lock null to x"), presumably it's waiting for the other instance to release the block. But what is taking the time - high CPU=20 queues or the fact that the other instance is waiting on something else?

Thanks!

Alec.
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http://www.freelists.org/webpage/oracle-l Received on Thu Oct 28 2004 - 13:51:42 CDT

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