Interview Questions - UNIX Memory Management

Kernel follows Round Robin scheme choosing a swap device among the multiple swap devices in Unix System V.

A Region is a continuous area of a process's address space (such as text, data and stack). The kernel in a "Region Table" that is local to the process maintains region. Regions are sharable among the process.

When Kernel swaps the process out of the primary memory, it performs the following:

1. Kernel decrements the Reference Count of each region of the process. If the reference count becomes zero, swaps the region out of the main memory,
2. Kernel allocates the space for the swapping process in the swap device,
3. Kernel locks the other swapping process while the current swapping operation is going on,
4. The Kernel saves the swap address of the region in the region table.

Process before swapping is residing in the primary memory in its original form. The regions (text, data and stack) may not be occupied fully by the process, there may be few empty slots in any of the regions and while swapping Kernel do not bother about the empty slots while swapping the process out.

After swapping the process resides in the swap (secondary memory) device. The regions swapped out will be present but only the occupied region slots but not the empty slots that were present before assigning.

While swapping the process once again into the main memory, the Kernel referring to the Process Memory Map, it assigns the main memory accordingly taking care of the empty slots in the regions.