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Synchronization

This example explores the need for synchronization in an operating system, using the scenario of two robots maintaining the milk inventory at a store. The robots must coordinate their actions to ensure efficient milk restocking.

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Synchronization

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  1. Synchronization Andy Wang Operating Systems COP 4610 / CGS 5765

  2. Motivating Example: Too Much Milk • Two robots are programmed to maintain the milk inventory at a store… • They are not aware of each other’s presence… Robot: Dumber Robot: Dumb

  3. Dumb 10:00 Look into fridge: Out of milk Dumber Motivating Example: Too Much Milk

  4. Dumb 10:00 Look into fridge: Out of milk 10:05 Head for the warehouse Dumber Motivating Example: Too Much Milk

  5. Dumb 10:05 Head for the warehouse Dumber 10:10 Look into fridge: Out of milk Motivating Example: Too Much Milk

  6. Dumb Dumber 10:10 Look into fridge: Out of milk 10:15 Head for the warehouse Motivating Example: Too Much Milk

  7. Dumb 10:20 Arrive with milk Dumber 10:15 Head for the warehouse Motivating Example: Too Much Milk

  8. Dumb 10:20 Arrive with milk Dumber 10:15 Head for the warehouse Motivating Example: Too Much Milk

  9. Dumb 10:20 Arrive with milk 10:25 Go party Dumber Motivating Example: Too Much Milk

  10. Dumb 10:20 Arrive with milk 10:25 Go party Dumber 10:30 Arrive with milk: “Uh oh…” Motivating Example: Too Much Milk

  11. Definitions • Synchronization: uses atomic operations to ensure cooperation among threads • Mutual exclusion: ensures 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

  12. 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

  13. Too Much Milk: Solution 1 • Two properties: • Only one robot will go get milk • Someone should go get the milk if needed • Basic idea of solution 1 • Leave a note (kind of like a lock) • Remove the note (kind of like a unlock) • Don’t go get milk if the note is around (wait)

  14. Too Much Milk: Solution 1 if (no milk) { if (no note) { // leave a note; // go get milk; // remove the note; } }

  15. Dumb 10:00 if (no milk) { Dumber Too Much Milk: Solution 1

  16. Dumb 10:00 if (no milk) { Dumber 10:01 if (no milk) { Too Much Milk: Solution 1

  17. Dumb 10:00 if (no milk) { Dumber 10:01 if (no milk) { 10:02 if (no note) { Too Much Milk: Solution 1

  18. Dumb 10:00 if (no milk) { 10:03 if (no note) { Dumber 10:01 if (no milk) { 10:02 if (no note) { Too Much Milk: Solution 1

  19. Dumb 10:00 if (no milk) { 10:03 if (no note) { 10:04 // leave a note Dumber 10:01 if (no milk) { 10:02 if (no note) { Too Much Milk: Solution 1

  20. Dumb 10:03 if (no note) { 10:04 // leave a note Dumber 10:01 if (no milk) { 10:02 if (no note) { 10:05 // leave a note Too Much Milk: Solution 1

  21. Dumb 10:03 if (no note) { 10:04 // leave a note 10:06 // go get milk Dumber 10:02 if (no note) { 10:05 // leave a note Too Much Milk: Solution 1

  22. Dumb 10:03 if (no note) { 10:04 // leave a note 10:06 // go get milk Dumber 10:05 // leave a note 10:07 // go get milk Too Much Milk: Solution 1

  23. Too Much Milk: Solution 2 • Okay…solution 1 does not work • The notes are posted too late… • What if both robots begin by leaving their own notes?

  24. Too Much Milk: Solution 2 // leave a note; if (no note from the other) { if (no milk) { // go get milk; } } // remove the note;

  25. Dumb 10:00 // leave a note Dumber Too Much Milk: Solution 2

  26. Dumb 10:00 // leave a note Dumber 10:01 // leave a note Too Much Milk: Solution 2

  27. Dumb 10:00 // leave a note 10:02 if (no note from Dumber) {…} Dumber 10:01 // leave a note Too Much Milk: Solution 2

  28. Dumb 10:00 // leave a note 10:02 if (no note from Dumber) {…} Dumber 10:01 // leave a note 10:03 if (no note from Dumb) {…} Too Much Milk: Solution 2

  29. Dumb 10:00 // leave a note 10:02 if (no note from Dumber) {…} 10:04 // remove the note Dumber 10:01 // leave a note 10:03 if (no note from Dumb) {…} Too Much Milk: Solution 2

  30. Dumb 10:00 // leave a note 10:02 if (no note from Dumber) {…} 10:04 // remove the note Dumber 10:01 // leave a note 10:03 if (no note from Dumb) {…} Too Much Milk: Solution 2

  31. Dumb 10:02 if (no note from Dumber) {…} 10:04 // remove the note Dumber 10:01 // leave a note 10:03 if (no note from Dumb) {…} 10:05 // remove the note Too Much Milk: Solution 2

  32. Dumb 10:02 if (no note from Dumber) {…} 10:04 // remove the note Dumber 10:01 // leave a note 10:03 if (no note from Dumb) {…} 10:05 // remove the note Too Much Milk: Solution 2

  33. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { if (no milk) { // go get milk } } // remove Dumber’s note Too Much Milk: Solution 3

  34. 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

  35. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { } // remove Dumber’s note Dumber Challenges Dumb: Case 1 Time

  36. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { } // remove Dumber’s note Dumber Challenges Dumb: Case 2 Time

  37. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { } // remove Dumber’s note Dumber Challenges Dumb: Case 3 Time

  38. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { if (no milk) { // go get milk } } // remove Dumber’s note Dumb Challenges Dumber: Case 1 Time

  39. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { } // remove Dumber’s note Dumb Challenges Dumber: Case 2 Time

  40. Dumb // leave Dumb’s note while (Dumber’s note) { }; if (no milk) { // go get milk } // remove Dumb’s note Dumber // leave Dumber’s note if (no Dumb’s note) { } // remove Dumber’s note Dumb Challenges Dumber: Case 3 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();

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