Lets start with basics
A process can spawn multiple threads with each thread executing performing different task concurrently. The advantages with threads is that they require less system resources and start easily when compared to processes.
Generally a process can run in 2 modes
- Kernel mode or system mode
- User mode:
Kernel mode : There is a 1-1 relationship between kernel and thread. Execution of process in kernel mode provides the thread with complete and unrestricted access to the underlying hardware. Thereby improving the performance of the process.
User mode: There is no dedicated kernel process and many to many relation ship exist between kernel and thread. The user mode is less expensive and support less concurrency compare to kernel mode. Scheduling is done by user scheduler
On AIX the variable AIXTHREAD_SCOPE specifies if Oracle processes will run in kernel mode or user mode. That is whether a thread has process wide contention(P) or system wide contention(S). The default is P or process wide contention.
Process wide Scope
When Oracle starts a thread with process wide scope, the following occurs:
1. No dedicated kernel thread, Many to Many relation ship exist between kernel and thread.
2. Placed in run queue for CPU intensive system
3. Subjected to time slicing by the user scheduler.
System Wide Scope
When Oracle starts a thread with system wide scope, the thread is bound to the kernel and scheduled by the kernel. There is a 1-1 relationship between kernel and thread. In other words, the kernel thread is not shared with any other user thread. As you are aware , execution of process in kernel mode provides the thread with complete and unrestricted access to the underlying hardware. Therefore improving the performance of the process.
Oracle recommends system wide scope.
References : IBM Documenation , Oracle Metalink