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Only the processes that are needed will be running for a particular instance.
Now Oracle can run in multi-threaded mode in Linux (Similar to Windows).

 ALTER SYSTEM SET threaded_execution=true SCOPE=SPFILE;

The database must be administered by an account that is authenticated through a password file.
OS authentication is no longer supported, must now login as SYS user using the password.
Different sessions will now have the same SPID but a different STID (Session Thread ID).

Oracle Base Article