Oracle database internals by Riyaj

Discussions about Oracle performance tuning, RAC, Oracle internal & E-business suite.

Posts Tagged ‘external table’

RAC, parallel query and udpsnoop

Posted by Riyaj Shamsudeen on June 20, 2009

I presented about various performance myths in my ‘battle of the nodes’ presentation. One of the myth was that how spawning parallel query slaves across multiple RAC instances can cause major bottleneck in the interconnect. In fact, that myth was direct result of a lessons learnt presentation from a client engagement. Client was suffering from performance issues with enormous global cache waits running in to 30+ms average response time for global cache CR traffic and crippling application performance. Essentially, their data warehouse queries were performing hundreds of parallel queries concurrently with slaves spawning across three node RAC instances.

Of course, I had to hide the client details and simplified using a test case to explain the myth. Looks like either a)my test case is bad or b) some sort of bug I encountered in version c) I made a mistake in my analysis somewhere. Most likely it is the last one :-(. Greg Rahn questioned that example and this topic deserves more research to understand this little bit further. At this point, I don’t have and database is in and so we will test this in


UDP is one of the protocol used for cache fusion traffic in RAC and it is the Oracle recommended protocol. In this article, UDP traffic size must be measured. Measuring Global cache traffic using AWR reports was not precise. So, I decided to use a dtrace tool kit tool:udpsnoop.d to measure the traffic between RAC nodes. There are two RAC nodes in this setup. You can read more about udpsnoop.d. That tool udpsnoop.d can be downloaded from dtrace toolkit . Output of this script is of the form:

PID        LADDR           LPORT           DR         RADDR           RPORT                 SIZE
---------- --------------- --------------- ---------- --------------- --------------- -----------
15393      38395           ->      40449                 8240

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