Page Replacement

• If we increase our degree of multiprogramming, we are over-allocating memory.
  • If we run six processes, each of which is ten pages in size but actually uses only five pages, we have higher CPU utilization and throughput, with ten frames to spare.
  • It is possible that each of these processes may suddenly try to use all ten of its pages resulting in a need for sixty frames.
• Further, consider that system memory is not used only for holding program pages. Buffers for I/O also consume a significant amount of memory. This use can increase the strain on memory-placement algorithms.
• Over-allocation of memory manifests itself as follows (see Fig. 9).
  • While a user process is executing, a page fault occurs.
  • The OS determines where the desired page is residing on the disk but then finds that there are no free frames on the free-frame list; all memory is in use.

  Figure 9: Need for page replacement.

  
• The OS could swap out a process, freeing all its frames and reducing the level of multiprogramming.