1. Computer Laboratory Practical non-blocking linked lists Tim Harris tim.harris@cl.cam.ac.uk Computer Laboratory

2. Overview: lists using CAS Insert 20: H10 30 T 20 30  20 10 30 20 30  20

3. Computer Laboratory Overview: lists using CAS Insert 20: H10 30 T 20 30  20 25 30  25 10 30 20 30  20 25 30  25 

4. Computer Laboratory Overview: lists using CAS Delete 10: H10 30 T 10  30 H10 30 10  30

5. Computer Laboratory Overview: lists using CAS Delete 10 & insert 20: H10 30 T 10  30 H10 30 10  30 20 30  20 H10 30 10  30 H10 30 10  30 20 30  20 

6. Computer Laboratory What can you do? Stronger primitives, e.g. DCAS H10 10  3030  XX H10 10  3030  XX 30 T

7. Computer Laboratory What can you do? Extra indirection: H10 30 T 30  1010  b 10d b 30 30  10 10d b c a H10 10  b a

8. Computer Laboratory Logical vs physical deletion Use a ‘spare’ bit to indicate logically deleted nodes: H10 30 T 20 10  30  30  30X  30  20  H10 30 20 10  30  30  30X  30  20 

9. Computer Laboratory H Storage management Must control memory re-use while threads hold pointers to it H10 30 T10 30

10. Computer Laboratory Storage management (2) Time A B C Ref counting, tracing GC, time-based re-use Add object to A’s deferred free list A finishes. C not complete: can’t free object yet Add object to C’s deferred free list C finishes A can now re-use the object it removed: B&C can’t have pointers to it Delayed threads can prevent any re-use

11. Computer Laboratory Storage management (3) Also need to consider visibility of updates H10 30 T 20 30  20 Write-write 10 30 20 30  20 Write-write

12. Computer Laboratory Storage management (4) …and re-ordering of dependent reads H10 30 T 20 10 30 while (val < seek) { p = p->next; val = p->val; }   val = ???

13. Computer Laboratory Random insert/delete Per-thread CPU-time / s Valois (ref-counted) New (ref-counted) Valois Number of threads Valois, ref-counting New, ref-counting Valois New Mutex

14. Computer Laboratory Random insert/delete (2) Per-thread CPU-time / s Valois (ref-counted) New (ref-counted) Valois Number of threads Valois, ref-counting New, ref-counting Valois New Mutex Valois New

15. Computer Laboratory Successful insertions Occurances Duration / cycles Mutex New Valois

16. Computer Laboratory Unsuccessful insertions Duration / cycles Mutex New Mutex Valois Occurances

17. Computer Laboratory Delete greater-than H10 30 T DeleteGE 20 10 30 20 Insert 20 30  T  T  TX  Insert 30   30?

18. Computer Laboratory Delete greater-than (2) H10 30 T Insert 20: 3020 T  20X 30 T 20 T  20X