Which segments to clean?

• Greedy policy

  • clean segments that are most fragmented

• Cost-benefit policy

  • value of cleaning a segment depends on:
    • cost: segment utilization
    • benefit: segment utilization and time to stay free
  • select a segment with highest benefit/cost
    (1-u) * age / (1 + u)
    

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Greedy policy - most fragmented segment cleaned first

cold segments stay around much longer

BUT - free space in cold segments are much valuable - they stay around longer

hot segment - data die quickly - so wait a little longer to get the free space even larger

how to find stability of a data in a segment:

assumption: older the data in a segment, longer it will remain unchanged

segment-usage table: # of live blocks + last modification time