Swapping

Swapping

• Keeping all processes in memory all the time requires a large amount of main memory. For execution of the process, it must be swapped.

• Swapping is method of temporarily removing inactive process/programs from the physical memory.

• Inactive program means, which is neither executing on the CPU, nor performing an I/O operation.

• Fig. 4.2.1 shows swapping

• When process P₂ send requests for an I/O operation, OS blocked P₂ process and will not return to the ready state for long period of time. Process P₂ is in blocked state. Process manager place the process P₂ in blocked state and inform to the memory manager regarding this action.

• Process manager moves a blocked process P₁ into the ready state and inform to memory manager. Process P₁ is loaded into main memory by memory manager when there is space available.

• Swapped out process is stored on the secondary storage disk. It contains executable image with code, data and stack.

• Relocation hardware supports for swapping. Because of address binding problem, swapping is difficult without relocation hardware.

• Round robin and priority based CPU scheduling algorithm uses the swapping concept.

• Time sharing system uses swapping. In time sharing, kernel can estimate when a program is likely to be scheduled next.

• A swapped out process will occupy same memory location when process swap in by OS. But this totally depends on address binding. If address binding is load time or assembly time, then swapped out process uses same memory location when swap-in. But execution-time binding uses different memory location.

• Secondary storage is required to implement the swapping technique.

• Content switching time increases when swapping increases. Major part of the swap time is transfer time. Swapping increases the operating system overhead due to the disk I/O involved.

• When swapping creates multiple holes in memory, it is possible to combine them all into one by moving all the processes to one side of memory. This technique is known as memory compaction.

• Compaction requires a lot of CPU time, so most operating systems not used this technique.