Two Mark Questions in Operating Systems / Interview Questions in Operating Systems
Swapping: Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.
Paging: Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory. It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.
Historic Unix uses Swapping - entire process is transferred to the main memory from the swap device, whereas the Unix System V uses Demand Paging - only the part of the process is moved to the main memory. Historic Unix uses one Swap Device and Unix System V allow multiple Swap Devices.
- It decides which process should reside in the main memory,
- Manages the parts of the virtual address space of a process which is non-core resident,
- Monitors the available main memory and periodically write the processes into the swap device to provide more processes fit in the main memory simultaneously.
A Map is an Array, which contains the addresses of the free space in the swap device that are allocatable resources, and the number of the resource units available there.
This allows First-Fit allocation of contiguous blocks of a resource. Initially the Map contains one entry - address (block offset from the starting of the swap area) and the total number of resources.
Kernel treats each unit of Map as a group of disk blocks. On the allocation and freeing of the resources Kernel updates the Map for accurate information.
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.