Lecture: Page Eviction

Limited Memory

• What should the OS do if processes want more memory when all frames are taken?
  • option 1: out of memory error, mobile OSes typically kill processes that overused memory to make space
  • option 2: pick a frame to evict, write the frame to a storage device, then re-allocate the frame
    • why do we need to write the data to disk?
    • do we always have to write the data out?
• Eviction Mechanisms:
  • Where to write data to?
  • swap partition: a section on disk that stores swapped out memory content
  • swap allocation: track free space inside the swap partition, often bitmap
  • update bookkeeping structures so we can find the data later
  • How to reflect the result of eviction?
  • remove the existing page to frame mapping
    • what if there are multiple pages mapped to the frame? (cow, shared memory)
    • struct core_map_entry tracks information about each frame
  • shootdown TLB if the evicted page is within the current process
• Eviction Policies: deciding which page to evict

Eviction Policies

• What pages should be our eviction candidates?
  • global policies: pages from all processes
  • pros and cons?
  • local policies: pages within the requesting process
  • pros and cons?
• Policies
  • FIFO:
    • evicts pages in the order that has been in memory the longest
    • what access pattern might work poorly for this?
    • Belady's anomaly (OSPP: Ch 9.5.6)
  • LRU:
    • evicts pages that were least recently used
    • how would you implement this?
    • what access pattern might work poorly for this?
  • Clock:
    • approximates LRU
    • uses the access bit inside a page table entry, bit set by hardware whenever the page is accessed (brought into TLB)
    • algorithm:
      
      1. track a clock hand (starting frame to look at for eviction)
      2. look at the frame at the clock hand, check the page table entry for the mapped page
      3. if pte's access bit is 0, select the page to evict, advance the clock hand to the next frame
      4. if pte's access bit is 1, clear the access bit to 0, shootdown TLB entry, advance the clock hand, and go to step 2
    • recall not all evicted pages need to be written out to swap, but clock doesn't care
  • Enhanced Clock (Second Chance):
    • clock but look at both access and dirty bit
    • if page is not dirty then we don't need to write it out (cheaper)
    • conditions:
      1. if access is 0 and dirty is 0, select page to evict, advance the clock hand
      2. if access is 1 and dirty is 0, clear access bit, advance the clock hand, keep searching
      3. if access is 0 and dirty is 1, clear dirty bit (track the dirty info elsewhere), advance the clock hand, keep searching
      4. if access is 1 and dirty is 1, clear access bit, advance the clock hand, keep searching