Synchronization

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Synchronization
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Motivating Example Too Much Milk

• Two robots are programmed to maintain the milk
  inventory at a store
• They are not aware of each others presence

Robot Dumber
Robot Dumb
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Motivating Example Too Much Milk

• Dumb
• 400 Look into fridge
• Out of milk

• Dumber

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Motivating Example Too Much Milk

• Dumb
• 400 Look into fridge
• Out of milk
• 405 Head for the warehouse

• Dumber

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Motivating Example Too Much Milk

• Dumb
• 405 Head for the warehouse

• Dumber
• 410 Look into fridge
• Out of milk

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Motivating Example Too Much Milk

• Dumb

• Dumber
• 410 Look into fridge
• Out of milk
• 415 Head for the warehouse

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Motivating Example Too Much Milk

• Dumb
• 420 Arrive with milk

• Dumber
• 415 Head for the warehouse

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Motivating Example Too Much Milk

• Dumb
• 420 Arrive with milk

• Dumber
• 415 Head for the warehouse

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Motivating Example Too Much Milk

• Dumb
• 420 Arrive with milk
• 425 Go party

• Dumber

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Motivating Example Too Much Milk

• Dumb
• 420 Arrive with milk
• 425 Go party

• Dumber
• 430 Arrive with milk Uh oh

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Definitions

• Synchronization use atomic operations to ensure
  cooperation among threads
• Mutual exclusion ensure one thread can do
  something without the interference of other
  threads
• Critical section a piece of code that only one
  thread can execute at a time

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More on Critical Section

• A lock prevents a thread from doing something
• A thread should lock before entering a critical
  section
• A thread should unlock when leaving the critical
  section
• A thread should wait if the critical section is
  locked
• Synchronization often involves waiting

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Too Much Milk Solution 1

• Two properties
• Someone should go get the milk if needed
• Only one robot will go get milk
• Basic idea of solution 1
• Leave a note (kind of like a lock)
• Remove the note (kind of like a unlock)
• Dont go get milk if the note is around (wait)

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Too Much Milk Solution 1

• if (no milk)
• if (no note)
• // leave a note
• // go get milk
• // remove the note

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Too Much Milk Solution 1

• Dumb
• 400 if (no milk)

• Dumber

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Too Much Milk Solution 1

• Dumb
• 400 if (no milk)

• Dumber
• 401 if (no milk)

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Too Much Milk Solution 1

• Dumb
• 400 if (no milk)

• Dumber
• 401 if (no milk)
• 402 if (no note)

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Too Much Milk Solution 1

• Dumb
• 400 if (no milk)
• 403 if (no note)

• Dumber
• 401 if (no milk)
• 402 if (no note)

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Too Much Milk Solution 1

• Dumb
• 400 if (no milk)
• 403 if (no note)
• 404 // leave a note

• Dumber
• 401 if (no milk)
• 402 if (no note)

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Too Much Milk Solution 1

• Dumb
• 403 if (no note)
• 404 // leave a note

• Dumber
• 401 if (no milk)
• 402 if (no note)
• 405 // leave a note

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Too Much Milk Solution 1

• Dumb
• 403 if (no note)
• 404 // leave a note
• 406 // go get milk

• Dumber
• 402 if (no note)
• 405 // leave a note

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Too Much Milk Solution 1

• Dumb
• 403 if (no note)
• 404 // leave a note
• 406 // go get milk

• Dumber
• 405 // leave a note
• 407 // go get milk

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Too Much Milk Solution 2

• Okaysolution 1 does not work
• The notes are posted too late
• What if both robots begin by leaving their own
  notes?

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Too Much Milk Solution 2

• // leave a note
• if (no note from the other)
• if (no milk)
• // go get milk
• // remove the note

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note

• Dumber

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note

• Dumber
• 401 // leave a note

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note
• 402 if (no note from Dumber)

• Dumber
• 401 // leave a note

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note
• 402 if (no note from Dumber)

• Dumber
• 401 // leave a note
• 403 if (no note from Dumb)

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note
• 402 if (no note from Dumber)
• 404 // remove the note

• Dumber
• 401 // leave a note
• 403 if (no note from Dumb)

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Too Much Milk Solution 2

• Dumb
• 400 // leave a note
• 402 if (no note from Dumber)
• 404 // remove the note

• Dumber
• 401 // leave a note
• 403 if (no note from Dumb)

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Too Much Milk Solution 2

• Dumb
• 402 if (no note from Dumber)
• 404 // remove the note

• Dumber
• 401 // leave a note
• 403 if (no note from Dumb)
• 405 // remove the note

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Too Much Milk Solution 2

• Dumb
• 402 if (no note from Dumber)
• 404 // remove the note

• Dumber
• 401 // leave a note
• 403 if (no note from Dumb)
• 405 // remove the note

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Too Much Milk Solution 3

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // go get milk
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• if (no milk)
• // go get milk
• // remove Dumbers note

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Too Much Milk Solution 3

• How do we verify the correctness of a solution?
• Test arbitrary interleaving of locking and
  checking locks
• In this case, leaving notes and checking notes

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Dumber Challenges Dumb Case 1

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // go get milk
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• // remove Dumbers note

Time
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Dumber Challenges Dumb Case 2

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // go get milk
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• // remove Dumbers note

Time
37
Dumber Challenges Dumb Case 3

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• // remove Dumbers note

Time
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Dumb Challenges Dumber Case 1

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• if (no milk)
• // go get milk
• // remove Dumbers note

Time
39
Dumb Challenges Dumber Case 2

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // go get milk
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• // remove Dumbers note

Time
40
Dumb Challenges Dumber Case 3

• Dumb
• // leave Dumbs note
• while (Dumbers note)
• if (no milk)
• // go get milk
• // remove Dumbs note

• Dumber
• // leave Dumbers note
• if (no Dumbs note)
• // remove Dumbers note

Time
41
Lessons Learned

• Although it works, Solution 3 is ugly
• Difficult to verify correctness
• Two threads have different code
• Difficult to generalize to N threads
• While Dumb is waiting, it consumes CPU time (busy
  waiting)
• More elegant with higher-level primitives
• lock?acquire()
• if (no milk) // go get milk
• lock?release()