Describe swapping technique in UNIX systems.

Swapping is used to control memory contention among processes. If there is excessively much memory contention, processes are swapped out until sufficient memory is available. Allocation of both main memory as well as swap space is done first fit. When the size of a process main memory image increases, a new piece of memory large enough for the whole image is allocated. The memory image is copied and the old memory is freed and the appropriate tables are updated. If no single piece of memory is large sufficient the process is swapped out such as it will be swapped back in with the new size.

Decisions as regards which process to swap in or out are made by the process scheduler, as well called as the swapper. The scheduler wakes up at least every 4 seconds to check for processes to be swapped in or out. There are confirm to avoid thrashing, fundamentally by not letting a process to be swapped out, previous to it has been in the memory for a certain amount of time.

If jobs require not be swapped out, the process table is searched to find a process deserving to be brought in. If there isn't enough memory available processes are swapped out until there.