Diffs

• P does a W

  • create twin, preserve it
  • do further W’s on original copy

• At barrier

  • compute diff
  • discard twin

• Notice: no communication has taken place so far

• The barrier mechanism

  • informs Q that P has modified the page and vice versa
  • P and Q’s copies are invalidated

• When P or Q references the page again

  • access fault
  • get diffs, update copies

• But what if overlapping modifications?

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