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Oracle 10g Determining The Setpoint for shared_servers

The shared_servers initialization parameter specifies the number of shared servers that you want to start, either at startup if it is in the initialization file, or at the time you issue the alter system set shared_servers command. Once shared server is started, Oracle Database 10g dynamically adjusts the number of shared servers based on the length of the request queue.

Generally, the number of shared servers stabilizes at a ratio of about one shared server for every ten client connections. For OLTP applications, the connections-to-servers ratio will be higher. The number of connections per shared server process might be higher when the rate of requests is low, or when the ratio of server usage to requests is low. On the other hand, in applications with a high rate of requests, or where the server usage-to-request ratio is high, the connections-to-server ratio could be lower than ten.

The value of shared_servers also indicates the minimum number of shared servers to be maintained, so that they cannot be marked for termination by PMON (process monitor) as the load on the shared server decreases. It can be used to prevent PMON from terminating, then restarting, shared servers due to coincidental fluctuations in load. Hence, this parameter helps to stabilize the load and minimize strain on the system.

If you know what the average load is like on your system, you can set shared_servers to an optimal value by configuring the max_shared_servers and shared_servers parameters, such that max_shared_servers is set to the value of maximum number of connections/10, while the shared_servers parameter is set to the value of (average number of connections/10. These settings can be adjusted up or down, depending on your actual run queue lengths and load types.

When shared_servers is not included in the initialization parameter file at database startup, but dispatchers is included specifying at least one dispatcher, then shared server is enabled. In this situation, where dispatchers is set and shared_servers is not, the default for shared_servers is one.

For the situation where neither shared_servers nor dispatchers are included in the initialization file, you cannot dynamically start the shared server by later using alter system to specify a setting for dispatchers. In

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The above text is an excerpt from "Oracle Database 10g New Features: Reference for Advanced Tuning and Administration", by Rampant TechPress.  Written by top Oracle experts, this book has a complete online code deport with ready to use scripts. 

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