by Martin McClure Correctness of a multi-threaded program is much more difficult to achieve than correctness of a single-threaded program, but in some situations we can't avoid using more than one thread, and therefore have to deal with the resulting complexity. In this talk, Martin will present some of the threading problems he's seen and solutions to those problems, with a focus on the additional problems that arise when threads are terminated.

1. Martin is getting
the projector
to work with
his laptop.

2. Threads,
Critical Sections,
and Termination
Martin McClure

3. Thread
vs.
Process

4. Shared
State

5. ...sigh

6. GemBuilder®
for
Smalltalk

7. GBS

8. Mapping
Dictionaries

9. ClientMap
• Identity Dictionary
•~2M keys
•~100K updates/s
•~(1-10)×100K lookups/s

11. 32-bit VW

12. 32-bit VW
214
hash values

15. Safe sharing of
this structure?

16. Semaphore

17. Semaphore
...performance?

18. Performance

19. "Premature
optimization is the
root of all evil."
--Donald Knuth

20. Simplicity
Clarity

21. Performance

22. Performance
5ms

23. Semaphore
...performance?

24. Time microsecondsToRun:
[1000000 timesRepeat: []]

25. Time microsecondsToRun:
[1000000 timesRepeat: []]
3ns-4ns

26. []
3ns-4ns

27. [sem critical: []]

28. [sem critical: []]
~70ns

29. [dict
delegateAtClient: key]

30. [dict
delegateAtClient: key]
~40ns

31. [sem critical:
[dict
delegateAtClient: key]]

32. [sem critical:
[dict
delegateAtClient: key]]
~130ns

33. Batched
semaphore
for modifying

34. No
semaphore
for lookup

35. Hash Bucket Lookup
index := linear search for key.
^index == 0
ifTrue: [nil]
ifFalse:
[value := self basicAt: index.
(keys at: index) == key
ifTrue: [value] ifFalse: [nil]]

36. Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

37. ...but there’s
a hole

38. Hash Bucket Lookup
index := linear search for key.
^index == 0
ifTrue: [nil]
ifFalse:
[value := self basicAt: index.
(keys at: index) == key
ifTrue: [value] ifFalse: [nil]]

39. Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

40. [“empty block”] ensure:
[“non-empty block”].

41. Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

42. Semaphore
critical: mutuallyExcludedBlock
self waitIfCurtailedSignal.
^mutuallyExcludedBlock
ensure: [self signal]

43. testTerminateInEnsure
| process count random delay |
random := Random new.
10 timesRepeat:
[process :=
[count := 0.
[] ensure:
[10 timesRepeat:
[count := count + 1.
1000000 timesRepeat: [12 factorial]].
count := count + 1]]
forkAt: Processor activeProcess priority - 1.
delay := (random next * 100) asInteger + 10. "avoid 0-ms delay"
(Delay forMilliseconds: delay) wait.
self assert: process isTerminated not.
process terminate.
process priority: Processor activeProcess priority + 1.
self
assert: process isTerminated;
assert: count equals: 11]

44. Terminating
in a
Critical Section

45. Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

46. Is this
the
best way?

47. Other Ways
• Semaphore
•#valueUninterruptably
•#valueUnpreemptively

48. Semaphore

49. Semaphore
•No termination
protection

50. Semaphore
•No termination
protection
•70ns

51. #valueUninterruptably

52. ~87ns
[]valueUninterruptably

53. #valueUnpreemptively

54. []valueUnpreemptively
~175ns

55. [] ensure: []

56. [] ensure: []
~15ns

57. Final
Thoughts

58. ?

59. Threads,
Critical Sections,
and Termination
Martin McClure