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2Q Algorithms. Fall 2004 CS 186 Review Session for hw1 9/13/04. Your ever friendly TA’s Paul Huang <cs186-ta@imail.eecs> Matt Denny <cs186-tb@imail.eecs> Murali Rangan <cs186-tc@imail.eecs>. Overview. LRU - Disadvantages 2Q – Enhanced LRU Simplified 2Q Full 2Q Q & A.

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2q algorithms

2Q Algorithms

Fall 2004 CS 186

Review Session for hw1

9/13/04

Your ever friendly TA’s

Paul Huang <cs186-ta@imail.eecs>

Matt Denny <cs186-tb@imail.eecs>

Murali Rangan <cs186-tc@imail.eecs>

overview
Overview
  • LRU - Disadvantages
  • 2Q – Enhanced LRU
  • Simplified 2Q
  • Full 2Q
  • Q & A
lru disadvantages
LRU - Disadvantages
  • Locality of reference
    • Same pages referred frequently (warm pages)
    • Example: 2, 1, 3, 2, 4, 2, 4, 1, 5, 6, 2, …
  • LRU takes advantage of this.
  • Disadvantage:
    • Dislodges warm pages if a long sequence of one time page references occur.
      • In the above ex, page 2 may get dislodged by the access pattern …, 4, 1, 5, 6, …
2q enhanced lru
2Q – Enhanced LRU
  • 2Q is enhancement over LRU
  • Manages pages in 2Q’s (FIFO and LRU)
  • Aims to retain warm pages as long as possible
  • Two variations
    • Simplified 2Q
    • Full 2Q
  • Efficient compared to LRU-2, GClock,etc
    • LRU-2, GClock - beyond the scope of this discussion
simplified 2q
Simplified 2Q
  • Pages in 2 queues (FIFO and LRU)
  • If a requested page
    • Is in FIFO (A1) => Move it to LRU head
    • Is in LRU (Am) => Move it to LRU head
    • If page fault
      • If free frame available, occupy it and add it to FIFO
      • Elif FIFO size > threshold (50% pool size)
        • Then first item in FIFO is victim
      • Else remove the tail item LRU, but add new page to FIFO (New pages ALWAYS go to FIFO)
simplified 2q contd1
Simplified 2Q (contd…)
  • Correlated references
    • A page receives several references for a short time and no references for a long time
      • Ex: 2, 3, 4, 1, 6, 7, 7, 6, 4, 4, 2, 3, 2, 2, 3,12, 15, …
  • Disadvantages
    • Evicts long term warm pages (ex 2, 3) to make room for short term correlated pages (ex 6, 7)
simplified 2q contd2
Simplified 2Q (contd…)
  • About the tuning threshold (chosen as 50%)
    • FIFO queue grows in spite of threshold as long as there are free frames.
    • FIFO threshold is checked only if there is a page fault and no free frames.
    • Larger the threshold => Greater the chance for FIFO to remain longer
    • Longer the FIFO => Greater the chance for some page in it to be re-referenced and promoted to LRU
simplified s2q
Simplified S2Q
  • About threshold (contd…)
  • The immediateness of re-referencing a page plays higher factor in promotion to LRU than total number of repetitions.
  • Ex: 2, 3, 3, 4, 4, 2, 5, 5, 7, 6, 2, 6, 7, 6, 8, 2
    • The chance for pages making it to LRU are:
    • (3, 4, 5) > (6) > (7) > (2) > 8
      • 3, 4, and 5 have highest chance because of the immediateness of their re-reference, even though they are only short term and referred only twice each.
      • 6 has lesser chance than (3, 4, 5) because the page re-reference is not as immediate as 3, 4, 5, even though it gets referred more.
      • Same argument for pages 7 and 2.
    • Threshold is the factor in determining how farther apart re-referenced pages will make it to LRU
    • But higher threshold also means smaller LRU as downside!
full 2q
Full 2Q
  • Addresses Correlated references
  • Pessimistic on re-referenced FIFO pages
    • Retains them in FIFO (A1in) itself
  • Remembers page ids of victimized FIFO pages as history (A1out – FIFO of page ids)
  • Puts a page in LRU only if it is seen in history list (A1out)
  • A1out max size is 50% of pool size
  • A1in threshold is 25% of pool size
f2q contd
F2Q (contd…)
  • If a requested page
    • Is in FIFO (A1in) => Do nothing (corr. ref.!)
    • Is in LRU (Am) => Move it to LRU head
    • If page fault
      • If free frame available, occupy it and add it to FIFO
      • Elif FIFO size > threshold (25% pool size)
        • Then first item in FIFO is victim,
          • Add page id of victim in A1out (50% pool size)
      • Else remove the tail item LRU, but add new page to FIFO (New pages ALWAYS go to FIFO)