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Monday, January 11, 2010

Performance of Demand Paging

Advantages of Demand Paging :
* Only loads pages that are demanded by the executing process.
* As there is more space in main memory, more processes can be loaded reducing context switching time which utilizes large amounts of resources.
* Less loading latency occurs at program start-up, as less information is accessed from secondary storage and less information is brought into main memory.
* Does not need extra hardware support than what paging needs, since protection fault can be used to get page fault.

Disadvantages of Demand Paging :
* Individual programs face extra latency when they access a page for the first time. So demand paging may have lower performance than anticipatory paging algorithms such as pre-paging.
* Programs running on low-cost, low-power embedded systems may not have a memory management unit that supports page replacement.
* Memory management with page replacement algorithms becomes slightly more complex.
* Possible security risks, including vulnerability to timing attacks.

Performance Of Demand Paging :
Let p be the probability of a page fault (0<=p<=1). We would expect p to be close to zero i.e. there will be only few page faults. The effective access time is then :
effective access time = (1-p) * ma + p * page fault time
To compute the effective access time, we must know how much time is needed to service a page fault. A page fault causes the following sequence to occur :
- Trap to the operating system.
- Save the user registers and process state.
- Determine that the interrupt was a page fault.
- Check that the page reference was legal and determine the location of the page on the disk.
- Issue a read from the disk to a free frame.
- While waiting, allocate the CPU to some other user.
- Interrupt from the disk.
- Save the registers and process state for the other user.
- Determine that the interrupt was from the disk.
- Correct the page table and other tables to show that the desired page is now in memory.
- Wait for the CPU to be allocated to this process again.
- Restore the user registers, process state, and new page table, then resume interrupted instruction.

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