Oracle Database Discussions

Hi,
yesterday I changed internet banking connections to MTS (shared server)
So, I changed the system to mix architecture (dedicated & shared both)
DB version is 10.2.0.3
OS version is AIX 5.3.5
Number of dispatchers are 2, each with 250 connections.
Shared_Servers=10
Max_Shared_Servers=40

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After 3 hours, I take the following error, no oracle process can be forked after in the OS.
At the time of problem, # of oracle processes in the OS was about 4000,
which is the OS limit for max processes a user can create.
But the number of sessions in the v$session was about 2000.
I also observed that SMON process turned to ZOMBIE (defunct) process.
It seems that MTS opens more oracle processes in the OS or turned some oracle processes to zombie.
But how can it be, since MTS does not create any new oracle processes except shared server process
in the OS.
When I turned back to dedicated server, everything works fine, that is, database sessions and OS oracle prosesses are almost the same.
Thanks,

ORA-27300: OS system dependent operation:fork failed with status: 11
ORA-27301: OS failure message: Resource temporarily unavailable
ORA-27302: failure occurred at: skgpspawn3