CS 3733 Operating Systems

• Working set of a process: The logical pages that a process needs to execute. All page replacement policies try to keep the working set of the processes in real memory.

• Global versus local allocation
  • Global allocation
    • All of the frames are candidates for replacement.
    • A process real memory allocation can grow at the expense of others.
    • Good memory utilization.

  • Local allocation
    • The replaced page belongs to the faulting process.
    • A process works within its allocation and can't affect others.
    • In real schemes local replacement is used with medium-term memory allocation to give a process experiencing a lot of page faults more frames to work with. This variation is called dynamic local allocation.

• VAX VMS Memory Management:
  • Keep a system list of free frames in a FIFO queue.
  • Keep a significant number of frames in the free list.
  • When a process must give up a page for replacement, the page is
  • put at the end of the system FIFO free list and the actual replaced page comes from the front of the list.
  • If a process references a page which it released to the free list,
  • it can get it back. Hence, the FIFO queue acts like a cache for the free pages.
  • Periodically the system uses the page fault rate of a process to increase or decrease the number of pages allocated to a particular process (within authorized limits).

• UNIX BSD 4.2
  • Old versions just had swapping
  • BSD 4.3 uses demand paging
  • A page marked invalid by the page replacement process, but still in table can be gotten back. (See below)
  • Replacement algorithm is Global Clock Least Recently Used.
    • Non-kernel memory is swept in circular fashion by a software clock hand.
    • When clock hand reaches a given frame, page table entry is located:
      • If already free, nothing is done.
      • If used by an I/O operation in progress, nothing is done.
      • If bit marked invalid in page table entry, page is freed.
      • If bit marked valid in page table entry, it is marked invalid, but reclaimable.
  • The LRU clock is the pagedaemon (process 2). It spends most of its time sleeping, but is awakened if the number of free frames falls below the threshold, lotsfree.
  • If load is too great, swapper swaps things out.

• Windows NT
  • Uses demand paging with clustering.
    When a thread gets a fault, the page manager loads the page plus a small number in the vicinity.
  • NT uses a dynamic local page allocation policy.
  • NT uses a FIFO page replacement policy. However, it caches the replaced pages in memory so that if they are referred to for a while after they are replaced, they can be gotten back.

Skill: Understand how paging might actually be implemented.