Memory allocation sequence to dialog work processes in SAP

What is the memory allocation sequence to dialog work processes in SAP? 

When does a work process go to PRIV mode? 

How to avoid or minimize work process going to PRIV mode ? 

What are the SAP parameters used to define initial roll area, extended memory, heap memory, roll area ?

Memory allocation sequence to dialog work processes in SAP :

Initially,a defined roll area is used.This roll area is defined by the SAP parameter ztta/roll_first.

Usually ztta/roll_first is set to 1 in SAP so that only necessary amount is allocated to roll memory.

If the memory from the initial roll area(i.e. ztta/roll_first) is not sufficient for the user context then comes 
extended memory.
Extended memory is used until the extended memory is full or until the user quota is reached Extended memory is defined by the parameter em/initial_size_MB and the user quota for dialog work process is defined by the parameter ztta/roll_extension_dia.

If this memory is also not sufficient, then The rest of the roll area is used. This roll area is defined by SAP parameter ztta/roll_area.

Once this is also fully occupied then
The system is forced to use local heap memory (Private memory). Then the work process goes into PRIV mode

Heap memory is available until one of the following occurs :
Either the limit of the heap memory for dialog work processes is reached (abap/heap_area_dia) or 
the entire heap memory of all work processes (abap/heap_area_total) for an application server reaches its limit.

Operating system limit for allocation of memory
The swap space in the host system is used up or the upper limit of the operating system address space is reached.

The memory allocation strategy for dialog work processes, aims to prevent work processes from allocating R/3 heap memory and thus entering PRIV mode.
When a work process enters PRIV mode, it remains connected to the user until the user ends the transaction. 
Most of the time, we should try to avoid the situation of work process going into PRIV mode for better performance of the SAP system. This can be done by optimally defining abap/heap_area_total parameter.