Sunday, September 15, 2013

Number of Session Vs. Processes

Relationship between sessions on a database and the number of processes.

The parameters SESSIONS and PROCESSES determine the size of two arrays in the SGA.

If you try to create more sessions on an instance than specified by the SESSIONS parameter, you will get an ORA-00018: maximum number of sessions exceeded.

If you try to create more processes on an instances than specified by the PROCESSES prameter, you wil get an ORA-00020: maximum number of processes exceeded.

The number of sessions and processes on your instance can been seen using a COUNT(*) against V$PROCESS and V$SESSION. Your job is to determine resonable values, so your users can continue
to work without getting neither ORA-18 nor ORA-20 during normal conditions. (A run away job, that keeps starting new sessions is not a normal condition, and it should be stopped by ORA-18 or ORA-20
whichever happens first).

You can join the two V$-views using the columns V$SESSION.PADDR and V$PROCESS.ADDR.

It is quite normal to see a 1:1 relationship between processes and sessions. However one OCI-program can create multiple sessions belonging to one process. Users of Oracle Portal
will experience this behaviour.

The default value for SESSIONS is 1.1*PROCESSES + 5.

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