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Performance Of Demand Paging In O/S

 As we Know  when we load the entire program in physical memory at program execution time, Known as paging. And when we load pages of program only as they are neede, known as demand paging and commonly used in virtual memory system. With demand paging, virtual memory pages are loaded only when they are demanded during program execution. Pages that that are never accessed are thus never loaded into physical memory (RAM).
  
So, demand paging can significantly affect the performance of a computer system. To see how demand paging affect the performance of computer system, let's compute the 'effective access time' for a demand-paged memory. For most computer systems, the memory-access time is denoted by ma, ranged from 10 to 200 nanoseconds. As long as we have no page faults, the effective access time is equal to the memory access time. But if a page fault occurs, we must first read the relevant page from disk and then access the desired word.

To complete the effective access time, we must know how much time needed to service a page fault. A page fault cause following sequence to occur :-
  • Trap to the operating system.
  • Save the user register and process state.
  • Determine that the interrupt was a page fault.
  • Determine the location of the page on the disk.
  • While waiting, allocate the CPU to some other user.
  • Recieve an interrupt from the disk I/O subsystem.
  • Save the registers and process state for the other user.
  • Determine that the interrupt was from the disk.
  • Correct the page table to show that the desired page is now in memory.
  • Wait for the CPU to be allocated to this process again.
  • Restore the user register, process state, and new pagetable, and then resume the interrupted instruction.
Not all of these steps are necessary in every case. In any case, we are facing with three major compounds of the page-fault service time :-
  1. Service the page-fault interrupt.
  2. Read in the page.
  3. Restart the process.
The first and third tasks can be, with careful coding, to several hundred instructions. These tasks may take from 1 to 100 microsecond.

An addiotional maspect of demand paging is the handling and ovarall use vof swap space. Disk I/O swap space is generally faster than thjat to the file system.
some systems attempt to limit the amount of swap space used through demand paging of binary files.

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