Demand paged memory management in OS

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Demand Paging :
The process of loading the page into memory on demand (whenever page fault occurs) is known as demand paging.
The process includes the following steps :
virtual_mem
  1. If CPU try to refer a page that is currently not available in the main memory, it generates an interrupt indicating memory access fault.
  2. The OS puts the interrupted process in a blocking state. For the execution to proceed the OS must bring the required page into the memory.
  3. The OS will search for the required page in the logical address space.
  4. The required page will be brought from logical address space to physical address space. The page replacement algorithms are used for the decision making of replacing the page in physical address space.
  5. The page table will updated accordingly.
  6. The signal will be sent to the CPU to continue the program execution and it will place the process back into ready state.
Hence whenever a page fault occurs these steps are followed by the operating system and the required page is brought into memory.
Advantages :
  • More processes may be maintained in the main memory: Because we are going to load only some of the pages of any particular process, there is room for more processes. This leads to more efficient utilization of the processor because it is more likely that at least one of the more numerous processes will be in the ready state at any particular time.
  • A process may be larger than all of main memory: One of the most fundamental restrictions in programming is lifted. A process larger than the main memory can be executed because of demand paging. The OS itself loads pages of a process in main memory as required.
  • It allows greater multiprogramming levels by using less of the available (primary) memory for each process.
Page Fault Service Time :
The time taken to service the page fault is called as page fault service time. The page fault service time includes the time taken to perform all the above six steps.
Let Main memory access time is: m
Page fault service time is: s
Page fault rate is : p
Then, Effective memory access time = (p*s) + (1-p)*m

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