Page Replacement Cont.

• Global vs. Local Allocation
  • Global replacement; process selects replacement frame from set of all frames; process can take frame from another
  • Local replacement each process selects only from its own set of allocated frames
• Frame locking; frames that belong to resident kernel, or are used for critical purposes, may be locked for improved performance.
• Page buffering; victim frames are grouped into two categories: those that hold unmodified (clean) pages and modified (dirty) pages
• Thrashing (see Fig. 8);

  Figure 8: Trashing.

  
  • The number of processes that are in the memory determines the multiprogramming (MP) level.
  • The effectiveness of virtual memory management is closely related to the MP level.
  • When there are just a few processes in memory, the possibility of processes being blocked and thus swapped out is higher.
  • When there are far too many processes (i.e., memory is overcommitted), the resident set of each process is smaller.
  • This leads to higher page fault frequency, causing the system to exhibit a behavior known as thrashing.
  • In other words, the system is spending its time moving pages in and out of memory and hardly doing anything useful.
  • process spends more time paging than executing
  • The only way to eliminate thrashing is to reduce the multiprogramming level by suspending one or more process(es).
  • Victim process(es) can be the: lowest priority process, faulting process, newest process, process with the smallest resident set, process with the largest resident set
  • Student analogy to thrashing: Too many courses!