1. The Joy of Synchronization Lecture 19, Nov 5

2. Administrivia Reminder: Schedule P3M1 with us Only 3 groups scheduled so far… Time slots running out...

3. Wheel of Time (god I hope not...) Last time: Multiprocessing intro: thread/process machine models Intro to Graphics2D: the basic graphics pipeline Geometry and rendering intro Today: Threading reprised More on geometry and affine transforms

4. The joy of synchronization

5. Synchronization Can’t control when/how long each thread runs Can ensure that threads don’t work on same data at same time

6. Synchronization Definition: critical section Segment of code that should not be invoked by >1 thread at a time

7. Synchronization Definition: Mutual exclusion Only 1 thread can be executing block (critical section) at a time

8. Synchronization Definition: Lock (monitor) Data struct used to ensure mutual exclusion w/in critical section

9. Kinds of exclusion Definition: enforced mutual exclusion You don’t have any choice. The system (OS, DB, etc.) forces mutex on the threads Definition: advisory mutual exclusion Everybody has to play nice together. It is up to the code to ensure that mutex is met.

10. Live Action Multithreading Demo

11. public class BoxObject { private int[] DataArray=new int[4]; public void twiddle(int c) { for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } BoxObject X=new BoxObject(); BoxObject Y=new BoxObject();

12. public class BoxObject { private int[] DataArray=new int[4]; public void twiddle(int c) { for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } BoxObject X=new BoxObject(); BoxObject Y=new BoxObject(); Thread A:Thread B: X.twiddle(3)Y.twiddle(5)

13. X.DataArray={ 0, 3, 6, 9 } Y.DataArray={ 0, 5, 10, 15 }

14. public class BoxObject { private int[] DataArray=new int[4]; public void twiddle(int c) { for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } BoxObject X=new BoxObject(); BoxObject Y=new BoxObject(); Thread A:Thread B: X.twiddle(3)X.twiddle(5)

15. X.DataArray={ 0, 3, 6, 9 } or, maybe... X.DataArray={ 0, 5, 10, 15 } or, maybe... X.DataArray={ 0, 5, 6, 9 } or even... X.DataArray={ 0, 3, 10, 9 }

16. public class BoxObject { private int[] DataArray=new int[4]; private Lock _l=new Lock(); }

17. public class BoxObject { private int[] DataArray=new int[4]; private Lock _l=new Lock(); public void twiddle(int c) { _l.retrieve(); for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } _l.release(); }

18. public class BoxObject { private int[] DataArray=new int[4]; private Lock _l=new Lock(); public void twiddle(int c) { _l.retrieve(); for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } _l.release(); } Thread A:Thread B: X.twiddle(3)Y.twiddle(5)

19. public class BoxObject { private int[] DataArray=new int[4]; private Lock _l=new Lock(); public void twiddle(int c) { _l.retrieve(); for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; } _l.release(); } Thread A:Thread B: X.twiddle(3)X.twiddle(5)

20. public class BoxObject { private int[] DataArray=new int[4]; private static int[] StaticData= new int[4]; private Lock _l=new Lock(); }

21. public class BoxObject { private int[] DataArray=new int[4]; private static int[] StaticData= new int[4]; private Lock _l=new Lock(); public void twiddle(int c) { _l.retrieve(); for (i=0;i<DataArray.length;++i) { DataArray[i]=c*i; StaticData[i]=-c*c*i*i; } _l.release(); }

22. Back to Java: Synchronization

23. The (dirty) truth of synchronized Java synchronized keyword only means: Get the lock before entering block Release lock when leaving block Trick is: which lock do you mean?

24. Context Switch

26. Back to Graphics